rtspsrc: Using multicast UDP has no relation to seekability

The transport has no relation to whether a media can be seeked. The
range response having a duration is the correct thing to check for.

Part-of: <gstreamer/gst-plugins-good!947>
31 jobs for !947 with backport-mr-939-into-1.18-fix-seekable-mcast in 31 minutes and 54 seconds (queued for 5 seconds)
merge request