Remove once_cell from public API (#2973)

## Motivation and Context
Addresses  7 in #2413 

## Description
This small PR removes uses of `once_cell` from public API. The
`http_versions` module in the `aws_smithy_http` crate was dead-code.

## Testing
Relies on existing tests in CI.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: ysaito1001 <awsaito@amazon.com>

CHANGELOG.next.toml

@@ -102,3 +102,15 @@ message = "Required members with @contextParam are now treated as client-side re
 references = ["smithy-rs#2964"]
 meta = { "breaking" = false, "tada" = false, "bug" = false, target = "client" }
 author = "rcoh"
+
+[[aws-sdk-rust]]
+message = "Remove `once_cell` from public API"
+references = ["smithy-rs#2973"]
+meta = { "breaking" = true, "tada" = false, "bug" = false }
+author = "ysaito1001"
+
+[[smithy-rs]]
+message = "Remove `once_cell` from public API"
+references = ["smithy-rs#2973"]
+meta = { "breaking" = true, "tada" = false, "bug" = false, "target" = "all" }
+author = "ysaito1001"

rust-runtime/aws-smithy-http/external-types.toml

@@ -26,9 +26,6 @@ allowed_external_types = [
     # TODO(https://github.com/awslabs/smithy-rs/issues/1193): Feature gate references to Tokio `File`
     "tokio::fs::file::File",
 
-    # TODO(https://github.com/awslabs/smithy-rs/issues/1193): Don't expose `once_cell` in public API
-    "once_cell::sync::Lazy",
-
     # TODO(https://github.com/awslabs/smithy-rs/issues/1193): Once tooling permits it, only allow the following types in the `event-stream` feature
     "aws_smithy_eventstream::*",
 ]

rust-runtime/aws-smithy-http/src/lib.rs

@@ -29,7 +29,6 @@ pub mod body;
 pub mod endpoint;
 pub mod header;
 pub mod http;
-pub mod http_versions;
 pub mod label;
 pub mod middleware;
 pub mod operation;