Remove "use strict" directives in extension tests

We compile with alwaysStrict so these are not needed

extensions/css-language-features/server/src/test/completion.test.ts

@@ -2,8 +2,6 @@
  *  Copyright (c) Microsoft Corporation. All rights reserved.
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
-'use strict';
-
 import 'mocha';
 import * as assert from 'assert';
 import * as path from 'path';