Cleanup the `Projects::MirrorsController#update_now`

The following discussion from !5484 (merged) should be addressed:

• @DouweM started a discussion:

  Since this doesn't call super, any changes that are made in CE don't actually apply here, which could result in bugs or even security issues being fixed in CE, and still being open in EE...

  I think we're better off splitting this method up. If not now, can you please create an issue?

  The same thing goes for update.