Remove flow-strict from polyfillPromise (#22048)

Summary:
Flow is broken for a bunch of people and has been for the last few days. For some reason this isn't causing any builds to fail which we will need to look into. For now, this stops the bleeding.

I *think* this is because of case sensitivity differenes between Mac and where all of our CI machines run.
```
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ Libraries/Core/polyfillPromise.js:21:33

Importing from an untyped module makes it any and is not safe! Did you mean to add // flow to the top of Promise?
(untyped-import)

     18│  * If you don't need these polyfills, don't use InitializeCore; just directly
     19│  * require the modules you need from InitializeCore for setup.
     20│  */
     21│ polyfillGlobal('Promise', () => require('Promise'));
     22│
```

Pull Request resolved: https://github.com/facebook/react-native/pull/22048

Reviewed By: shergin

Differential Revision: D12918418

Pulled By: TheSavior

fbshipit-source-id: 9d9db1b3686eab906244180325c12ffcce92be8c

Libraries/Core/polyfillPromise.js

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @flow strict-local
+ * @flow
  * @format
  */
 'use strict';