rtspsrc: drop stream-start message posted by the internal udp sink(s)

See gst-plugins-bad#1368 (closed)

Edit: I have confirmed that stream-start is posted by rtspsrc's internal udp sinks and that it works all by accident because rtspsrc has 2 internal udp sinks (instead of 1 in rtpsrc). The first one posts stream-start too early and rtspsrc waits for the second stream-start from the other sink. However, this stream-start is cleared while rtspsrc changes state, so when the second sink posts stream-start, rtspsrc waits for the first one again to post it... This never happens, so the message is never forwarded.