Remove telnetd

The telnetd codebase is unmaintained and has a number of quality
issues. Telnet has been largely supplanted by ssh. If needed, a port is
available (net/freebsd-telnetd), but a more maintained implementation
should be prefered.

While the telnet client suffers from the same issues, it is deemed
to be of lower risk and is required to connect to legacy devices, so
it remains.

Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D36620