remove reference to pop3, telnet and rsh Makefiles

The pop3, telnet and rsh/rcp support was removed from the tree in e55b0d0ca5038a8101276a593ffbb6be4c27c8d0. Delete the corresponding Makefiles so autoconf doesn't try to look for them.

This is needed to run `./autogen.sh` on Fedora 20 (GNU Autoconf 2.69)