Disabling the Nether/End by default

[LogicParrot]

Multiworld travel is too unstable. It will surely crash the server after several teleports. I propose that we disable Nether portals and End portals by default by blanking out the default world's LinkedWorlds section.

I just want the default Cuberite settings to be as stable as possible.
Is everyone fine with that? I am willing to make the pull request.