Fix project deletion when there is a export available

Project deletions were failing with "Can't modify frozen hash" because:

1. Project#remove_exports was called in the after_destroy hook
2. This would remove the file and update ImportExportUpload
3. ImportExportUpload#save would attempt to write to a destroyed model

To avoid this, we just check if ImportExportUpload has been destroyed
before attempting to save it.

This would have a side effect of not running after_commit hooks to delete the
repository on disk, making it impossible to delete the project entirely.

Closes #52362