mac80211: remove tx_sync

When the station state callback was added, this
was no longer needed in theory. With the iwlwifi
changes to remove use of it landing, we can kill
the entire tx-sync framework again, RIP.

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