Revert "SDK: Deprecate the ability to implement RegExp and RegExpMatch."

This reverts commit 9f2a26e.

Reason for revert: b/449079357

Original change's description:
> SDK: Deprecate the ability to implement RegExp and RegExpMatch.
>
> Work towards #55608
>
> Change-Id: Ibf666bb8366997ead0b5ae60840854059381effd
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451981
> Reviewed-by: Martin Kustermann <[email protected]>
> Reviewed-by: Stephen Adams <[email protected]>
> Reviewed-by: Lasse Nielsen <[email protected]>
> Commit-Queue: Samuel Rawlins <[email protected]>

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I6a69728e044b594f1d9e16fbb50308a3f5550f28
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/453221
Commit-Queue: Martin Kustermann <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>
Auto-Submit: Mayank Patke <[email protected]>

Author: fishythefish

sdk/lib/core/regexp.dart

@@ -213,10 +213,6 @@ part of "dart:core";
 /// when the regexp does not find a match.
 /// Several guides to [improving the performance of regular expressions](https://www.google.com/search?q=performance+of+regular+expressions)
 /// exist on the internet. Use these as inspirations, too.
-@Deprecated.implement(
-  "This class will become 'final' in a future release. "
-  "'Pattern' may be a more appropriate interface to implement.",
-)
 abstract interface class RegExp implements Pattern {
   /// Constructs a regular expression.
   ///
@@ -472,10 +468,6 @@ abstract interface class RegExp implements Pattern {
 ///   // This is the second message.
 /// }
 /// ```
-@Deprecated.implement(
-  "This class will become 'final' in a future release."
-  "'Match' may be a more appropriate interface to implement.",
-)
 abstract interface class RegExpMatch implements Match {
   /// The string captured by the named capture group [name].
   ///