A previous patch has caused rtpfunnel to output twcc-related information downstream, however this leaked into upstream negotiation (through funnel->srccaps), causing payloader to negotiate twcc caps even when not prompted to do so by the user. Fix this by only enforcing that upstream sends us application/x-rtp caps as was the case originally. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1278>