[Elements.migrate] Migrate RelevanceComputer

Change-Id: I0654cc8b95f1100f3d4884b3d6a8220c604d3513
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/396383
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Keerti Parthasarathy <[email protected]>

Author: keertip

pkg/analysis_server/analyzer_use_new_elements.txt

@@ -34,7 +34,6 @@ lib/src/services/completion/dart/feature_computer.dart
 lib/src/services/completion/dart/identifier_helper.dart
 lib/src/services/completion/dart/in_scope_completion_pass.dart
 lib/src/services/completion/dart/not_imported_completion_pass.dart
-lib/src/services/completion/dart/relevance_computer.dart
 lib/src/services/completion/dart/utilities.dart
 lib/src/services/completion/dart/visibility_tracker.dart
 lib/src/services/completion/postfix/postfix_completion.dart