mt76: remove variable set but not used

The code that uses variable queued has been removed,
and "mt76_is_usb(dev) ? q->ndesc - q->queued : q->queued"
didn't do anything, so all they should be removed as well.

Eliminate the following clang warnings:
drivers/net/wireless/mediatek/mt76/debugfs.c:77:9: warning: variable
‘queued’ set but not used.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 2d8be76c ("mt76: debugfs: improve queue node readability")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>