Remove external config evaluation script

[EvanBacon]

Why

• Resolves https://twitter.com/notbrent/status/1304277925278019585
• Slower computers struggle to dynamically evaluate a config from an external script.

How

• Removed the external script for config reading in favor of parsing the string contents of the config file via @babel/core and evaluating it as a string.
• I also removed 2 seemingly unused deep clone / evaluation calls, this should make reading a bit faster too.
  1. It's not needed on a static config because static configs don't support JS functions.
  2. It's unified in the shared evalConfig method.
• Removed an unused FS existence check.

Test Plan

• Tested locally that the contents of the config can still be updated during expo start. Ensured the dynamic behavior was used and Constants.manifest.name could be updated by reloading the app (⌘+r).

[EvanBacon]

This change might break #2220 by essentially reverting the issue they had initially by default. cc @RedMickey see test failures https://github.com/expo/expo-cli/pull/2625/checks?check_run_id=1101429349

[brentvatne]

https://twitter.com/benawad/status/1304985643173515275

[EvanBacon]

lmfao ¯_(ツ)_/¯