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/
PidginandFinch:ThePimpin' Penguin IM Clients That'reGoodfortheSoul
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