Revert "selinux: add a skb_owned_by() hook"

This reverts commit ca10b9e9.

No longer needed after commit eb8895de
("tcp: tcp_make_synack() should use sock_wmalloc")

When under SYNFLOOD, we build lot of SYNACK and hit false sharing
because of multiple modifications done on sk_listener->sk_wmem_alloc

Since tcp_make_synack() uses sock_wmalloc(), there is no need
to call skb_set_owner_w() again, as this adds two atomic operations.

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