JingleMediaRTP: use jingle_session_peer_has_cap()
JingleMediaRTP: use jingle_session_peer_has_cap()

The body of content_has_cap() is basically identical to the body of
gabble_jingle_session_peer_has_cap(), so let's delete the former.

mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble