Drop TODO item regarding audio/video calls

Drop TODO item regarding audio/video calls

Nathan is planning on signing up for a paid plan with Slack
temporarily (over the period of Mystery Hunt) specifically to get
access to Slack video calls. These are already fully integrated
(allowing a call to everyone who belongs to a channel) which means
that that will allow for a call to everyone working on a given
puzzle. So I don't think we'll need any specific work to integrate
Turbot with this approach.