fix: simplify logic for HTTP/1.1 REST fallback option (#55)

* fix: simplify logic for HTTP/1.1 REST fallback option

For the `fallback` parameter, all values considered as `true`
in Boolean context will enable HTTP/1.1 REST fallback,
since the other fallback transport, proto over HTTP, is
removed from `google-gax` v4.

PiperOrigin-RevId: 559812260

Source-Link: googleapis/googleapis@6a6fd29

Source-Link: https://github.com/googleapis/googleapis-gen/commit/56c16657e7a59122b1da94771a9ef40989c282c0
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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

src/v2/migration_service_client.ts

@@ -90,16 +90,15 @@ export class MigrationServiceClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
-   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   * @param {boolean} [options.fallback] - Use HTTP/1.1 REST mode.
    *     For more information, please check the
    *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    * @param {gax} [gaxInstance]: loaded instance of `google-gax`. Useful if you
    *     need to avoid loading the default gRPC version and want to use the fallback
    *     HTTP implementation. Load only fallback version and pass it to the constructor:
    *     ```
    *     const gax = require('google-gax/build/src/fallback'); // avoids loading google-gax with gRPC
-   *     const client = new MigrationServiceClient({fallback: 'rest'}, gax);
+   *     const client = new MigrationServiceClient({fallback: true}, gax);
    *     ```
    */
   constructor(
@@ -162,7 +161,7 @@ export class MigrationServiceClient {
     }
     if (!opts.fallback) {
       clientHeader.push(`grpc/${this._gaxGrpc.grpcVersion}`);
-    } else if (opts.fallback === 'rest') {
+    } else {
       clientHeader.push(`rest/${this._gaxGrpc.grpcVersion}`);
     }
     if (opts.libName && opts.libVersion) {

src/v2alpha/migration_service_client.ts

@@ -90,16 +90,15 @@ export class MigrationServiceClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
-   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   * @param {boolean} [options.fallback] - Use HTTP/1.1 REST mode.
    *     For more information, please check the
    *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    * @param {gax} [gaxInstance]: loaded instance of `google-gax`. Useful if you
    *     need to avoid loading the default gRPC version and want to use the fallback
    *     HTTP implementation. Load only fallback version and pass it to the constructor:
    *     ```
    *     const gax = require('google-gax/build/src/fallback'); // avoids loading google-gax with gRPC
-   *     const client = new MigrationServiceClient({fallback: 'rest'}, gax);
+   *     const client = new MigrationServiceClient({fallback: true}, gax);
    *     ```
    */
   constructor(
@@ -162,7 +161,7 @@ export class MigrationServiceClient {
     }
     if (!opts.fallback) {
       clientHeader.push(`grpc/${this._gaxGrpc.grpcVersion}`);
-    } else if (opts.fallback === 'rest') {
+    } else {
       clientHeader.push(`rest/${this._gaxGrpc.grpcVersion}`);
     }
     if (opts.libName && opts.libVersion) {

src/v2alpha/sql_translation_service_client.ts

@@ -88,16 +88,15 @@ export class SqlTranslationServiceClient {
    *     API remote host.
    * @param {gax.ClientConfig} [options.clientConfig] - Client configuration override.
    *     Follows the structure of {@link gapicConfig}.
-   * @param {boolean | "rest"} [options.fallback] - Use HTTP fallback mode.
-   *     Pass "rest" to use HTTP/1.1 REST API instead of gRPC.
+   * @param {boolean} [options.fallback] - Use HTTP/1.1 REST mode.
    *     For more information, please check the
    *     {@link https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md#http11-rest-api-mode documentation}.
    * @param {gax} [gaxInstance]: loaded instance of `google-gax`. Useful if you
    *     need to avoid loading the default gRPC version and want to use the fallback
    *     HTTP implementation. Load only fallback version and pass it to the constructor:
    *     ```
    *     const gax = require('google-gax/build/src/fallback'); // avoids loading google-gax with gRPC
-   *     const client = new SqlTranslationServiceClient({fallback: 'rest'}, gax);
+   *     const client = new SqlTranslationServiceClient({fallback: true}, gax);
    *     ```
    */
   constructor(
@@ -161,7 +160,7 @@ export class SqlTranslationServiceClient {
     }
     if (!opts.fallback) {
       clientHeader.push(`grpc/${this._gaxGrpc.grpcVersion}`);
-    } else if (opts.fallback === 'rest') {
+    } else {
       clientHeader.push(`rest/${this._gaxGrpc.grpcVersion}`);
     }
     if (opts.libName && opts.libVersion) {