tuntap: remove unnecessary sk_receive_queue length check during xmit

After commit 1576d986 ("tun: switch to use skb array for tx"),
sk_receive_queue was not used any more. So remove the uncessary
sk_receive_queue length check during xmit.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>