mt76: mt7921: removed unused definitions in mcu.h

Get rid of the following definitions in mt7921/mcu.h since they are not
actually used:
- MT7921_WTBL_UPDATE_MAX_SIZE
- MT7921_STA_UPDATE_MAX_SIZE
- MT7921_WTBL_UPDATE_BA_SIZE

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