Remove unused message for index operator
Remove unused message for index operator

This message was moved to a shared location in
https://dart-review.googlesource.com/c/sdk/+/130733

Change-Id: Iee0a396389aa75000fb3b0bae974da26ecd6ddcf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138046
Commit-Queue: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Auto-Submit: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@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