mt76x0: remove unused mt76x0_tx_status routine

Remove no longer used mt76x0_tx_status routine since
mt76x0 driver uses mt76-usb utility routines to report tx-feedbacks

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>