remove redundant experiment
remove redundant experiment

Since this package is opted-in, the experiment is no longer needed.

Change-Id: Ie5b855cc7b180ebcc30a3a4cb4001bbdc82c9c39
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/171622
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Phil Quitslund <3faecbdd7b7be83f493f037bf1d597e6bb9d0848@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