mac80211: remove unused don't-encrypt flag

When MFP is disabled, action frames will not
be encrypted since they are management frames
and the only management frames that can then
be encrypted are authentication frames.

Therefore, setting the don't-encrypt flag on
action frames is unnecessary.

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