Remove out-of-tree platform tests

[empyrical]

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 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 - 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.

Changelog

[General] [Removed] - Removed Out-of-Tree platform tests (functionality is tested in the RN-CLI repo now)

Test Plan

No regressions have been noted.

[pull-bot]

	Warnings
⚠️	🔒 package.json - Changes were made to package.json. This will require a manual import by a Facebook employee.

Generated by 🚫 dangerJS against 1db4b2a

[cpojer]

OMG this makes me so happy. Thank you.

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @empyrical in b27d1d3.

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