wifi: mt76: mt7996: remove mt7996_mcu_set_pm()

Currently using BSS_INFO_PS command will sometimes cause packet drop in
hw rx queue.
Temporarily remove this function until finding the cause.

Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>