Nuke custom release version functionality.

Official repos won't likely have more than one version for long, if at
all. Also nuke 3rd part repos since it's too complex to say "I want
libzmq3 and python-zmq to be from official backports, but salt-master
and salt-common to be from my custom repo", which is a valid use case.
It makes more sense to just support official backport repos for
everything.