dccp/tcp: Remove an unused argument in inet_csk_listen_start().

The commit 1295e2cf ("inet: minor optimization for backlog setting in
listen(2)") added change so that sk_max_ack_backlog is initialised earlier
in inet_dccp_listen() and inet_listen().  Since then, we no longer use
backlog in inet_csk_listen_start(), so let's remove it.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Reviewed-by: Richard Sailer <richard_siegfried@systemli.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>