tcp: remove unused hash_size from struct tcp_out_options

tcp: remove unused hash_size from struct tcp_out_options

hash_size is declared but never read. The MD5 path always uses a
fixed size of 16, and the TCP-AO path uses tcp_ao_maclen().

This closes a 7-byte hole and reduces the struct size from 96 to
88 bytes.

Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Keita Morisaki <kmta1236@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260307051619.51685-1-kmta1236@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>