net/tls: remove a duplicate function prototype

Remove one of the two instances of the function prototype for
tls_validate_xmit_skb().

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Boris Pismenny <borisp@nvidia.com>
Cc: Aviad Yehezkel <aviadye@nvidia.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>