Kill the LPR GTK+ backend

[geofft]

At some point we should probably drop our gtkrc file to add the LPR backend to the GTK+ printing dialog, and go back to the upstream behavior of just having the list of CUPS printers and not letting you type lpr command lines.

[jdreed]

I'm not sure about this. As long as we have arbitrary queues out there, we should at least let people print to them, even if we don't support them. The primary problem with the LPR backend is the printing wrappers' complexity, and that will be fixed when we drop support for LPRng.