[messages] Remove broken message_coverage_test.dart.

Removes the tests in `message_coverage_test.dart`, which have been
failing since 2017. These tests were part of a previous effort to
unify analyzer and CFE error codes, and are no longer needed. Removing
them will make it easier to clean up and refactor the logic for
handling shared error codes.

Change-Id: I6a6a696405dbbab1934f39294c89c0ed22c23395
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451860
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/analyzer/test/src/fasta/message_coverage_test.dart

@@ -5,14 +5,12 @@
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'ast_builder_test.dart' as ast_builder;
-import 'message_coverage_test.dart' as message_coverage;
 import 'recovery/test_all.dart' as recovery;
 import 'token_utils_test.dart' as token_utils;
 
 main() {
   defineReflectiveSuite(() {
     ast_builder.main();
-    message_coverage.main();
     recovery.main();
     token_utils.main();
   }, name: 'fasta');