mt76: Remove set but not used variables 'pid' and 'final_mpdu'

Fixes gcc '-Wunused-but-set-variable' warnings:

drivers/net/wireless/mediatek/mt76/mt7603/mac.c: In function mt7603_fill_txs:
drivers/net/wireless/mediatek/mt76/mt7603/mac.c:969:5: warning: variable pid set but not used [-Wunused-but-set-variable]
drivers/net/wireless/mediatek/mt76/mt7603/mac.c:961:7: warning: variable final_mpdu set but not used [-Wunused-but-set-variable]
drivers/net/wireless/mediatek/mt76/mt7615/mac.c: In function mt7615_fill_txs:
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:555:5: warning: variable pid set but not used [-Wunused-but-set-variable]
drivers/net/wireless/mediatek/mt76/mt7615/mac.c:552:19: warning: variable final_mpdu set but not used [-Wunused-but-set-variable]

They are never used, so can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>