[nnbd_migration] Hide hint insertion links after the migration is applied.

Change-Id: If89b16ec1c4e26e4aa1fb03fc262075c75eac526
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138389
Commit-Queue: Mike Fairhurst <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: MichaelRFairhurst