Revert "rtnetlink: add guard for RTNL"

This reverts commit 464eb03c.

Once we have a per-netns RTNL, we won't use guard(rtnl).

Also, there's no users for now.

  $ grep -rnI "guard(rtnl" || true
  $

Suggested-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/netdev/CANn89i+KoYzUH+VPLdGmLABYf5y4TW0hrM4UAeQQJ9AREty0iw@mail.gmail.com/


Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>