Jest setup: Stop polyfilling Promise.

It looks like this line was introduced in 3ff3987, in 2015, and it
has remained in a similar form since then. I haven't found any
explanation for it.

At jestjs/jest#10221 [1], a core Jest maintainer says,

"""
As an aside, one should never replace `global.Promise` [...]. E.g.
when using `async-await` you will always get the native `Promise`
regardless of the value of `global.Promise`.
"""

jestjs/jest#10221 is one issue this line has raised, for anyone
using the latest features of Jest to test async code in their React
Native projects.

[1] jestjs/jest#10221 (comment)

Fixes: facebook#29303

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.requestAnimationFrame = function(callback) {