mt76: remove unused variable q

Kernel test robot reported warning:

   drivers/net/wireless/mediatek/mt76/tx.c: In function
'mt76_txq_schedule':
>> drivers/net/wireless/mediatek/mt76/tx.c:499:21: warning: variable 'q'
>> set but not used [-Wunused-but-set-variable]
     499 |  struct mt76_queue *q;
         |                     ^

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1607542617-4005-1-git-send-email-jrdr.linux@gmail.com