mac80211: remove rate_control_clear

"Clearing" the rate control algorithm is pointless, none of
the algorithms actually uses this operation and it's not even
invoked properly for all channel switching. Also, there's no
need to since rate control algorithms work per station.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>