MediaFactory: Remove unused session_chans map
MediaFactory: Remove unused session_chans map

I guess this is left over from before the Jingle refactoring. Nothing
was ever added to session_chans, so this was all dead code.

Reviewed-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>

Ziemin/telepathy-gabble,jku/telepathy-gabble,jku/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,community-ssu/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble,jku/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble