remove CompletionCache and obsolete tests, and cleanup imports

pkg/analysis_server/test/domain_completion_test.dart

Index: pkg/analysis_server/test/domain_completion_test.dart
diff --git a/pkg/analysis_server/test/domain_completion_test.dart b/pkg/analysis_server/test/domain_completion_test.dart
index 45eb82cbd3322f308ea882c9febc7217b38969cc..958b9528ab5c85722619def36da4693312be40b7 100644
--- a/pkg/analysis_server/test/domain_completion_test.dart
+++ b/pkg/analysis_server/test/domain_completion_test.dart
@@ -16,6 +16,7 @@ import 'package:analysis_server/src/domain_completion.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
     show AnalysisRequest, CompletionRequest, CompletionResult;
+import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/completion_manager.dart';
 import 'package:analysis_server/src/services/completion/dart/contribution_sorter.dart';
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
@@ -793,10 +794,6 @@ class B extends A {m() {^}}
   }
 }
 
-class MockCache extends CompletionCache {
-  MockCache(AnalysisContext context, Source source) : super(context, source);
-}
-
 class MockCompletionManager implements CompletionManager {
   final AnalysisContext context;
   final Source source;