Remove ChannelMixin
Remove ChannelMixin

Was supposed to be deleted in 62c0c11

streamlink/streamlink-twitch-gui,chhe/livestreamer-twitch-gui,streamlink/streamlink-twitch-gui,chhe/livestreamer-twitch-gui,streamlink/streamlink-twitch-gui,chhe/livestreamer-twitch-gui,bastimeyer/livestreamer-twitch-gui,bastimeyer/livestreamer-twitch-gui,bastimeyer/livestreamer-twitch-gui