Remove "use strict" directives in extension tests

We compile with alwaysStrict so these are not needed

extensions/html-language-features/server/src/test/folding.test.ts

@@ -3,8 +3,6 @@
  *  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 { TextDocument } from 'vscode-languageserver';