[CQ] enable unnecessary_ignore

Change-Id: Iec50385ec1ecf56ed3056a0a844adcfee71ac09e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/407502
Reviewed-by: Brian Wilkerson <[email protected]>
Auto-Submit: Phil Quitslund <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: pq

pkg/analyzer_utilities/analysis_options.yaml

@@ -18,6 +18,7 @@ linter:
     - prefer_single_quotes
     - unawaited_futures
     - unnecessary_breaks
+    - unnecessary_ignore
     - unnecessary_library_directive
     - unnecessary_parenthesis
     - unreachable_from_main