Remove feature flag for has-async-alt attr

This patch removes the feature flag for @hasAsyncAlternative since it's
already protected by the experimental concurrency flag and will go in
with the concurrency features.

This fixes one of Doug's comments on
https://github.com/apple/swift/pull/36027.

lib/Frontend/CompilerInvocation.cpp

@@ -713,9 +713,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
-  Opts.EnableExperimentalHasAsyncAlternative |=
-      Args.hasArg(OPT_experimental_has_async_alternative_attribute);
-
   return HadError || UnsupportedOS || UnsupportedArch;
 }