RN: Delete isEmpty

Summary:
Deletes `isEmpty` because there are no more references to it in `react-native`.

Changelog:
[Internal]

Reviewed By: zackargyle, fred2028

Differential Revision: D17884310

fbshipit-source-id: 0554aee4044452b6c04f638f1ad762025eecd929

Libraries/vendor/core/isEmpty.js

@@ -1,29 +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 strict
- * @format
- */
-
-'use strict';
-
-/**
- * Mimics empty from PHP.
- */
-function isEmpty(obj: mixed): boolean {
-  if (Array.isArray(obj)) {
-    return obj.length === 0;
-  } else if (typeof obj === 'object') {
-    for (const i in obj) {
-      return false;
-    }
-    return true;
-  } else {
-    return !obj;
-  }
-}
-
-module.exports = isEmpty;