headers: Remove some left-over license text in include/uapi/linux/netfilter/

When the SPDX-License-Identifier tag has been added, the corresponding
license text has not been removed.

Remove it now.

Also, in xt_connmark.h, move the copyright text at the top of the file
which is a much more common pattern.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Florian Westphal <fw@strlen.de>