net/tcp: Use static_branch_tcp_{md5,ao} to drop ifdefs

It's possible to clean-up some ifdefs by hiding that
tcp_{md5,ao}_needed static branch is defined and compiled only
under related configs, since commit 4c8530dc ("net/tcp: Only produce
AO/MD5 logs if there are any keys").

Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>