Elements. Remove ElementResolverTest.

It is really old, we have better and more tests now.

Change-Id: I6c33ce2ffa4c13dc15f2065ba4be3774a38eb60c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/398960
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov