rtpfunnel: fix extmap handling on accept-caps

Follow-up on 97d83056, don't take extmap into account when checking if a sinkpad can accept caps.

I must have been lucky in my firefox testing then, and always entered the code path with audio getting negotiated first, thus not failing the is_subset check when srccaps had been negotiated as application/x-rtp, and an accept-caps query was made for the video caps with a defined extmap.