McdConnection: remove support for calling RequestChannel
McdConnection: remove support for calling RequestChannel

We could never really support it properly anyway, due to deficiencies in
the pre-Requests D-Bus API.

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control