Clean up legacy CSSBackgroundDrawable.java and enablNewBackgroundAndBorderDrawables featureflag (#53534)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53534

BackgroundDrawable and BorderDrawable have already substituted CSSBackgroundDrawable en every Android surface.
- Deleting CSSBackgroundDrawable.java and its callsites
- Deleting enableNewBackgroundAndDrawable featureflag

Just cleaning up what at this point is just dead code.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D81330969

fbshipit-source-id: bcf66ec8d3225802432ae1d93a2b26ea65cfcda0