mt76: remove offchannel check in tx scheduling

tx queues are already disabled by mac80211 during scanning or other
off-channel activity. There is no need to repeat the check in mt76,
since scheduled queues are selected by mac80211 as well.

Signed-off-by: Balakrishna Bandi <b.balakrishna@globaledgesoft.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>