mac80211: remove dead code

ieee80211_offchannel_enable_all_ps function is no longer used
and looks like its logic is extensively handled in
ieee80211_offchannel_stop_vifs

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>