RN: Unnecessary $FlowFixMe for jest.now() (#51665)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51665

This type suppression comment is no longer necessary.

Changelog:
[Internal]

Reviewed By: SamChou19815

Differential Revision: D75546017

fbshipit-source-id: 8c1f6a4260f0ca83bcba200bf550746270578534

packages/react-native/jest/setup.js

@@ -77,7 +77,6 @@ Object.defineProperties(global, {
     configurable: true,
     enumerable: true,
     value(callback: number => void): TimeoutID {
-      // $FlowFixMe[prop-missing] - JestObjectType is incomplete.
       return setTimeout(() => callback(jest.now()), 0);
     },
     writable: true,