Stop polyfilling Promise in Jest tests (facebook#34659)

Summary:
Pull Request resolved: facebook#34659

We've used this Promise polyfill in Jest setup since at least 2015 ([`3ff3987`](facebook@3ff3987)), when native Promise implementations were either non-existent or new and unstable. We no longer need it.

It causes issues with "modern" timers in Jest, as documented in:
 - facebook#29303
 - jestjs/jest#10221

It can also obscure real issues due to its default silent handling of uncaught rejections, eg: D39418412.

Changelog:
[General][Changed] - Don't polyfill Promise in Jest setup

Reviewed By: huntie

Differential Revision: D39417597

fbshipit-source-id: d12433ed66c06a402632c2e1d525aad112ef9b0c

jest/setup.js

@@ -21,7 +21,6 @@ global.performance = {
   now: jest.fn(Date.now),
 };
 
-global.Promise = jest.requireActual('promise');
 global.regeneratorRuntime = jest.requireActual('regenerator-runtime/runtime');
 global.window = global;