Summary:
This pull request removes the tests for out-of-tree platforms and the dependency on the package `react-native-dummy` from the React Native repo.

The logic that this was meant to test was moved to the React Native CLI repo, and [it is being tested there](https://github.com/react-native-community/cli/blob/827daa4c1687f8d17eb7df46e1a2ec25640c7797/packages/cli/src/tools/config/__tests__/index-test.js#L125-L152) as well making this a bit redundant at this point.

The dependency on `react-native-dummy` was also an issue, because when the file structure under `Libraries/` changes, bundler errors could crop up if there wasn't an update made to the file structure of `react-native-dummy/Libraries/`.

This was an issue when [`TabBarIOS` was removed](https://github.com/facebook/react-native/commit/02697291ff41ddfac5b85d886e9cafa0261c8b98#diff-b9cfc7f2cdf78a7f4b91a753d10865a2) - `react-native-dummy` was causing Haste errors and was stopping the commit from being able to land, and I needed to cut a new version just for it. With Lean Core, I expect more issues like this happening in the future.

[General] [Removed] - Removed Out-of-Tree platform tests (functionality is tested in the RN-CLI repo now)
Pull Request resolved: https://github.com/facebook/react-native/pull/24536

Differential Revision: D15063320

Pulled By: cpojer

fbshipit-source-id: 2a0467bed326b286623fa3cfa4e0bb6959b66b78