Remove explicit EnableString.non_nullable in BazelWorkspacePackage.
Remove explicit EnableString.non_nullable in BazelWorkspacePackage.

It is already enabled by default in the latest language version.

Change-Id: I91feabd596c35410890601ae82c446be5ea6ad3d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213116
Reviewed-by: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@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