ping: Remove a vestigial notdef

It was once a function on 4.3BSD, pr_type() [1], used to convert an ICMP
"type" field to a printable string. In 4.4BSD it was superseded by
pr_icmph() [2].

NetBSD [3] and OpenBSD [4] have already removed it.

[1]: https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD/usr/src/etc/ping.c
[2]: https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.4BSD/usr/src/sbin/ping/ping.c
[3]: https://github.com/NetBSD/src/commit/203dfd34867991fd002f747d74a96f26ae80d41c
[4]: https://github.com/openbsd/src/commit/9bbbbbb75d24e3d166ddd0cb6cb4417b78561309

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37247