netinet*: remove dead code from TCP, UDP, SCTP control input

Now these functions are called only from icmp*_input(). The pointer
to the ICMP data is never NULL and cmd has a limited set of values.

In the past the functions were demultiplexing control messages from
ICMP layer, as well as internally generated events. In the latter
case the the pointer to IP would be NULL.

Differential revision: https://reviews.freebsd.org/D36729