wifi: mt76: Remove unused inline function mt76_wcid_mask_test()

All uses of mt76_wcid_mask_test() have
been removed since commit 8950a62f ("mt76: get rid of
mt76_wcid_hw routine"), so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>