Delete duplicate Focus.js (#15540)

This file was renamed to the lower case version, but as Mac filesystems are typically case insensitive, it was never removed.

packages/react-events/Focus.js

@@ -1,14 +0,0 @@
-/**
- * Copyright (c) Facebook, Inc. and its affiliates.
- *
- * This source code is licensed under the MIT license found in the
- * LICENSE file in the root directory of this source tree.
- *
- * @flow
- */
-
-'use strict';
-
-const Focus = require('./src/Focus');
-
-module.exports = Focus.default || Focus;