mt76x0: remove unused routines for usb_buff alloc/free

Remove the following unused routines for usb buffer
allocation/free/submit:
- mt76x0_usb_alloc_buf
- mt76x0_usb_free_buf
- mt76x0_usb_submit_buf

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>