Clean up unused internal Symbol API.
Clean up unused internal Symbol API.

Change-Id: I9287130074d0a183985b53e16d0c31b49750e012
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/205660
Reviewed-by: Erik Ernst <251bb01623bfda2cd9151e339a6e783ca432874c@google.com>
Reviewed-by: Lasse R.H. Nielsen <80295e4ef244a07779f93eaa95d582bad8e316a5@google.com>
Commit-Queue: Mayank Patke <e0355b78204b67a55a952941c68c8f1b40235938@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk