mac80211: remove unnecessary iflist_mtx locking

The radar detection code changed a few times, and due to
the changes some iflist_mtx locking stayed in that isn't
actually necessary - remove it.

One version of the code needed it because an AP interface's
VLAN list was changed to use this, but then we moved the
list handling outside of the chanctx handling and thus the
locking was no longer needed.

Tested-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>