GStreamerSharp: Remove unneeded AudioCdRipper.OnNativeMimeType method

The corresponding actions are already implemented in the
RefreshTrackMimeType method.
GStreamerSharp: Rename AudioCdRipper.OnNativeFinished to OnFinished

The name was copied from the native backend, but there's nothing native
about this method anymore.
GStreamerSharp: Fix log statement in AudioCdRipper.OnBusMessage