tcp: remove redundant assigment to snd_cwnd

Not sure how this got in here. git blame says the second assignment was
added in 3a9a57f6, but that commit also removed the first assignment.

Signed-off-by: Theodore Dubois <tblodt@icloud.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>