sdp: Skip source-specific caps fields when creating an SDP media from caps

sdp: Skip source-specific caps fields when creating an SDP media from caps

Regression from https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2132

We parse the source-specific fields from the SDP and put it into the
caps, but when converting caps into an SDP again this would need special
handling. By default it would end up as part of the fmtp field, which is
simply wrong.

Automatically putting it into the caps and SDP will need some more work.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4344>