To undo the pppoeconf settings isn't very easy. If you use the pppoeconf settings you can connect to the Internet without connecting via network manager. So if there is a case that you want to undo this settings, without reinstalling the whole operating system, I have a little tutorial.
1) open console
2) get root:
3) edit nm-system-settings.conf
You have to write managed=true under [ifupdown]
4) edit interfaces
This file must look like this:
iface lo inet loopback
iface eth0 dhcp
5) Restart your computer
PS: If there are some questions, make a comment.