mac80211: Remove invalid flag operations in mesh TSF synchronization

mesh_sync_offset_adjust_tbtt() implements Extensible synchronization
framework ([1] 13.13.2 Extensible synchronization framework). It shall
not operate the flag "TBTT Adjusting subfield" ([1] 8.4.2.100.8 Mesh
Capability), since it is used only for MBCA ([1] 13.13.4 Mesh beacon
collision avoidance, see 13.13.4.4.3 TBTT scanning and adjustment
procedures for detail). So this patch remove the flag operations.

[1] IEEE Std 802.11 2012

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
[remove adjusting_tbtt entirely, since it's now unused]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>