conference: improve audio-only handling

1. When a audio-only participant leaves, the video mixer is updated
and all audio-only streams are now removed.
2. If a audio-only calls try to join a swarm-call, it was hanging
due to a bad number of medias. So ignore video for audio-only calls.

GitLab: #917
Change-Id: Ifdd66e2ab138c078ef9e9c1402a07a6ae6a36b00