kcm: Unbreak opening "Remove Connection" dialog
the platform theme told us a native dialog isn't available; not using native dialog
So with our platform theme this dialog shows up as an in-scene popup, it barely fits into the narrow column, but at least it allows removing connections once again.
Interestingly, in KF5 version a native dialog window shows up, though it's not really modal — main window gets dimmed but remains interactive.