Remove Danger.js workflow and React native bots package (#55385)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55385

Now that all Danger.js checks have been migrated to native GitHub Actions in the `annotate-pr.yml` workflow, this removes the deprecated Danger infrastructure:

- Deletes `.github/workflows/danger-pr.yml` workflow
- Deletes `private/react-native-bots/dangerfile.js`
- Removes `danger` npm dependency from `react-native-bots/package.json`

The functionality previously provided by Danger.js is now handled by:
- **API diff detection**: `diff-js-api-changes` action
- **PR body validation**: `validatePRBody.js` script (summary, test plan, changelog checks)
- **Branch targeting**: `checkBranchTarget.js` script (validates target branch, adds "Pick Request" label)
- **PR commenting**: `post-pr-comment` action

This simplifies the CI pipeline by removing the third-party Danger dependency and consolidating PR annotation logic into maintainable GitHub Actions workflows.

Changelog: [Internal]

Reviewed By: huntie

Differential Revision: D91695886

fbshipit-source-id: c88001ef75d16c4709c7972add141db0df3b5a30