Remove unused setupExtensionWatcher (#4904)

### WHY are these changes introduced?

Removes the extension bundler watcher and its tests as they are no longer needed after migrating to the new AppWatcher flow.

### WHAT is this pull request doing?

Deletes `bundler.ts` and `bundler.test.ts` files which were responsible for watching and rebuilding extensions during development. This functionality has been replaced by the new AppWatcher flow.

### How to test your changes?
Nothing to test, this PR just removes dead code.

### Measuring impact

- [x] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

### Checklist

- [x] I've considered possible cross-platform impacts (Mac, Linux, Windows)
- [x] I've considered possible [documentation](https://shopify.dev) changes