Remove entire PurpleDebugUiInterface and implementations
The `purple_debug*` functions are routed through GLib's logging interface, and the UIs capture logs from there, so there's no need for this interface any more.
Testing Done:
Opened Debug Window and saw logs appearing there.
Reviewed at https://reviews.imfreedom.org/r/1074/
[Desktop Entry]
Name=Pidgin
GenericName=Internet Messenger
Comment=Chat over IM. Supports Jabber/XMPP, and more