chore: code quality improvements and cleanup

[Programlog]

Summary

This PR contains small code quality improvements and cleanup across the React codebase.

Changes

1. Remove obsolete error code 355 for useOpaqueIdentifier

   • This error was for a feature that was never released
   • The error message itself contained a TODO: "This feature was never released so we should be able to remove this error from the map"
   • Confirmed no other references to this error code exist in the codebase

2. Improve @deprecated documentation for noEmit compiler option

   • Clarify the migration path to use outputMode instead
   • Updated terminology from "Forget" to "the compiler" for consistency
   • Made the deprecation notice more actionable

3. Remove stale TODO comment from ReactNative.js shim

   • The TODO suggested deleting the "legacy renderer" but this shim is still required for backward compatibility
   • Removing the misleading comment prevents confusion

Test Plan

These are documentation and dead code removal changes that don't affect runtime behavior:

• Error code 355 was for an unreleased feature and has no usages
• The noEmit option behavior is unchanged, only JSDoc improved
• The ReactNative.js shim behavior is unchanged, only comment removed

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