Remove "use strict" directives in extension tests

We compile with alwaysStrict so these are not needed

extensions/vscode-api-tests/src/singlefolder-tests/window.test.ts

@@ -3,8 +3,6 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
-'use strict';
-
 import * as assert from 'assert';
 import { workspace, window, commands, ViewColumn, TextEditorViewColumnChangeEvent, Uri, Selection, Position, CancellationTokenSource, TextEditorSelectionChangeKind, Terminal } from 'vscode';
 import { join } from 'path';