mac80211: remove module handling from rate control ops

mac80211: remove module handling from rate control ops

There's not a single rate control algorithm actually in
a separate module where the module refcount would be
required. Similarly, there's no specific rate control
module.

Therefore, all the module handling code in rate control
is really just dead code, so remove it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>