chore(android): remove obsolete ProGuard rules

[floyd-soomgo]

Summary

• Remove the redundant DefaultReactHostDelegate keep rule.
  • DefaultReactHostDelegate implements ReactHostDelegate, so its private jsBundleLoader is already covered by the following generic keep rule.
• Remove the obsolete Nimbus dontwarn rule.
  • CodePush no longer depends on Nimbus for code signing, and this consumer rule could suppress unrelated warnings in host apps.

Verification

• git diff --check