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.
Merge pull request #62 from ktdreyer/rm-old-makefile-references

remove reference to pop3, telnet and rsh Makefiles