sdp: garbage collect sdp_ctlinput

The pr_ctlinput method was a feature of IPv4/IPv6 with exception of
pfctlinput(), which broadcasted a call to pr_ctlinput on all protocols
ever registered statically or with pf_proto_register(). Now that
this broadcast call is gone, the only protocols that get their
pr_ctlinput ever called are those that have registered itselves with
ipproto_register() or ip6proto_register().

It is entirely possible that code deleted now was dead code from very
beginning. Just a copy-paste from TCP.

Reviewed by: rstone
Differential revision: https://reviews.freebsd.org/D36208