Back out the PurpleProtocol->login change that returned a PurpleConnection

This remains to be a good idea, but this implementation caused more issues than
it solved. I'm working on that better implementation, but we need to revert all
of this first, otherwise the new review request is going to be massive.

This reverts commits 3f96e2b2b03d, d567f89d6a9b, and 6ef5bb284651. These were
backed out manually due to some other commits that made automative backouts
difficult.

Testing Done:
Connected a demo, ircv3, and xmpp accounts without issue.

Reviewed at https://reviews.imfreedom.org/r/1937/