mac80211: remove message on receiving unexpected unencrypted frames

Some people are getting this message a lot, and we have traced it to
broken access points that much too often send completely empty frames
(all bytes zeroed, which they shouldn't do at all.)

Since we cannot do anything about such frames in any case except the
special case where we're debugging an AP, just remove the message.

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