mt76: remove no longer used routine declarations

Remove following routine declarations that are no longer used
after commit cfca5f693c5d ("mt76usb: remove usb_mcu.c"):

- mt76u_mcu_complete_urb
- mt76u_deinit
- mt76u_buf_free
- mt76u_submit_urb

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>