mt76x2: remove leftover function declatarions

Remove following function declarations since they have been moved
to mt76x02-lib module:
- mt76x2_sta_add
- mt76x2_sta_remove
- mt76x2_remove_interface
- mt76x2_conf_tx
- mt76x2_txq_init

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