[RNMobile] Remove ReactAztecTextInputShadowNodeFork

[mchowning]

Description

The changes that required the ReactTextInputShadowNodeFork class are now included in theReactTextInputShadowNode class since we've updated to React Native 0.64, so this PR is just removing the no longer needed "Fork" class. There should be no behavior changes in the app.

Additional context for why this code was originally introduced can be found here.

How has this been tested?

1. Run the demo app
2. Make sure that there has not been a regression in the padding around RichText inputs (see Remove extra inner padding on text blocks wordpress-mobile/gutenberg-mobile#992)

Types of changes

This is a refactoring that should result in no behavior changes.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[AmandaRiu]

Tested on Android and iOS and padding looks good! 👍 LGTM