Simplify JavaScriptCore opt-out instructions

[jvaronenka]

Current instructions were no longer correct - just switching the options would make the application crash. Thus, I removed detailed instructions for switching back to JavaScriptCore from Hermes by linking to the other repository, which has correct instructions.

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[Simek]

Thanks for the updates @jvaronenka! 👍

Last thing - can you accept the CLA so we can merge those changes in?