vaapidecode: remove gst_vaapidecode_internal_flush()
vaapidecode: remove gst_vaapidecode_internal_flush()

As gst_vaapidecode_finish() is the only callee of
gst_vaapidecode_internal_flush(), it is better to inline it.

https://bugzilla.gnome.org/show_bug.cgi?id=768652

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi