mac80211: remove fragmentation offload functionality

There's no driver that actually does fragmentation on the
device, and the callback is buggy (when it returns an error,
mac80211's fragmentation status is changed so reading the
frag threshold from userspace reads the new value despite
the error). Let's just remove it, if we really find some
hardware supporting it we can add it back later.

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