mt76x0: remove unused stat work_queue

Remove unused tx_status workqueue since now tx feedbacks are
processed by mt76-usb layer

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>