Remove deprecated flags

@bbatsov commented on https://github.com/bquorning/rubocop/commit/526dc81eb1608284f4cb735574b2df3d167ba65f that “They have been deprecated for about an year now and 1.0 is definitely not in sight.”

Not only have I removed the `--emacs` and `--silent` options completely, I also removed the “infrastructure” for deprecated options (`#ignore_dropped_options`, `#convert_deprecated_options` and `#deprecate` methods).

The build will pass once #1537 is merged and I have rebased.