Remove prepareESM Rollup build step entirely.

Since we aren't using Rollup to apply the invariant transform anymore, it
turns out we don't need this build step at all. Instead, the CJS bundles
will be generated directly from the modules generated by tsc and processed
by the invariant transform.

config/rollup.config.js

@@ -25,27 +25,6 @@ const external = [
   'zen-observable',
 ];
 
-function prepareESM(input, outputDir) {
-  return {
-    input,
-    external,
-    output: {
-      dir: outputDir,
-      format: 'esm',
-      sourcemap: true,
-    },
-    // The purpose of this job is to ensure each `./dist` ESM file is run
-    // through the `invariantPlugin`, with any resulting changes added
-    // directly back into each ESM file. By setting `preserveModules`
-    // to `true`, we're making sure Rollup doesn't attempt to create a single
-    // combined ESM bundle with the final result of running this job.
-    preserveModules: true,
-    plugins: [
-      nodeResolve(),
-    ],
-  };
-}
-
 function prepareCJS(input, output) {
   return {
     input,
@@ -140,7 +119,6 @@ function prepareBundle({
 }
 
 export default [
-  prepareESM(packageJson.module, distDir),
   prepareCJS(packageJson.module, packageJson.main),
   prepareCJSMinified(packageJson.main),
   ...entryPoints.map(prepareBundle),