[TCP]: Remove superflushious skb == write_queue_tail() check

Needed can only be more strict than what was checked by the
earlier common case check for non-tail skbs, thus
cwnd_len <= needed will never match in that case anyway.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>