basesink: Remove unused fields and always use the buffer timestamp difference for calculating the QoS proportion
basesink: Remove unused fields and always use the buffer timestamp difference for calculating the QoS proportion

The buffer timestamps are only hints and more often than not have
nothing to do with reality.

https://bugzilla.gnome.org/show_bug.cgi?id=771306

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer