netfilter: remove nf_ipv6_ops and use direct function calls

netfilter: remove nf_ipv6_ops and use direct function calls

As IPv6 is built-in only, nf_ipv6_ops can be removed completely as it is
not longer necessary.

Convert all nf_ipv6_ops usage to direct function calls instead. In
addition, remove the ipv6_netfilter_init/fini() functions as they are
not necessary any longer.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://patch.msgid.link/20260325120928.15848-12-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>