Allow disabling `YoutubeDlUpdateScheduler`-job if youtube-dl is not being used

Describe the problem to be solved

We do not allow HTTP imports in our PeerTube instance and we also don't allow our PeerTube instance to invoke web requests to github.com . We therefore get the error "Cannot update youtube-dl" (along with the very long HTTP response error details) whenever the YoutubeDlUpdateScheduler is running.

Describe the solution you would like

It would be great if PeerTube had a flag to disable the YoutubeDlUpdateScheduler-job. Maybe it could even look at the existing flags (import.videos.http.enabled, ...?) and disable the scheduler if all relevant flags indicate that youtube-dl is not being used.