Remove the non-driver versions of several tests

pkg/analysis_server/test/services/completion/dart/library_prefix_contributor_test.dart

Index: pkg/analysis_server/test/services/completion/dart/library_prefix_contributor_test.dart
diff --git a/pkg/analysis_server/test/services/completion/dart/library_prefix_contributor_test.dart b/pkg/analysis_server/test/services/completion/dart/library_prefix_contributor_test.dart
index 78bd338965d6fc128738b16ad659550ab64e0e4f..7010cb5aa2d1ae6990c3fca4264bd42c4bc94f55 100644
--- a/pkg/analysis_server/test/services/completion/dart/library_prefix_contributor_test.dart
+++ b/pkg/analysis_server/test/services/completion/dart/library_prefix_contributor_test.dart
@@ -15,12 +15,14 @@ import 'completion_contributor_util.dart';
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(LibraryPrefixContributorTest);
-    defineReflectiveTests(LibraryPrefixContributorTest_Driver);
   });
 }
 
 @reflectiveTest
 class LibraryPrefixContributorTest extends DartCompletionContributorTest {
+  @override
+  bool get enableNewAnalysisDriver => true;
+
   void assertSuggestLibraryPrefixes(List<String> expectedPrefixes) {
     for (String prefix in expectedPrefixes) {
       CompletionSuggestion cs = assertSuggest(prefix,
@@ -376,9 +378,3 @@ main() {new ^ String x = "hello";}''');
     assertNoSuggestions();
   }
 }
-
-@reflectiveTest
-class LibraryPrefixContributorTest_Driver extends LibraryPrefixContributorTest {
-  @override
-  bool get enableNewAnalysisDriver => true;
-}