Destroy mod-created CVars when 'restart' is invoked

Getting my feet wet with working on ZDoom-- apologies if I'm way off base here.

Addresses [this seemingly minor issue](http://forum.zdoom.org/viewtopic.php?f=2&t=45603), where CVars are not destroyed when `restart` is invoked from the console. My solution was to delete CVars flagged with CVAR_MOD, which seems to be the intended behavior.