client: never call gst_rtsp_media_unprepare, let gst_rtsp_media_finalize do it
client: never call gst_rtsp_media_unprepare, let gst_rtsp_media_finalize do it

Calling gst_rtsp_media_unprepare breaks shared medias. Just unref
GstRTSPMedia instances and let gst_rtsp_media_finalize unprepare when a media
isn't being used anymore.

Lachann/gst-rtsp-server,barisdemiray/gst-rtsp-server,hsteinhaus/gst-rtsp-server,thaytan/gst-rtsp-server,barisdemiray/gst-rtsp-server,hsteinhaus/gst-rtsp-server,surround-io/gst-rtsp-server,thaytan/gst-rtsp-server,surround-io/gst-rtsp-server,surround-io/gst-rtsp-server,hsteinhaus/gst-rtsp-server,barisdemiray/gst-rtsp-server,Lachann/gst-rtsp-server,thaytan/gst-rtsp-server,Lachann/gst-rtsp-server