ipv6: remove dynamic ICMPv6 sender registration infrastructure

ipv6: remove dynamic ICMPv6 sender registration infrastructure

As IPv6 is built-in only, there is no need to maintain the sender
registration infrastructure used to allow built-in subsystems to send
ICMPv6 messages when IPv6 was compiled as a module.

Drop the registration mechanism and the __icmpv6_send() sender
implementation. While icmpv6_send() users could be converted to
icmp6_send() that doesn't seems necessary as none of them are using the
force_saddr parameter.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://patch.msgid.link/20260325120928.15848-5-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>