Historically, sourcegraph.com has been the only instance. It was connected to GitHub.com and GitLab.com only.
Configuration should be as simple as possible, and we wanted everyone to try it on any repo. So public repos were added on-demand when browsed from these code hosts.

Since, dotcom is no longer the only instance, and this is a special case that only exists for sourcegraph.com.
This causes a bunch of additional complexity and various extra code paths that we don't test well enough today.

We want to make dotcom simpler to understand, so we've made the decision to disable that feature, and instead we will maintain a list of repositories that we have on the instance.
We already disallowed several repos half a year ago, by restricting size of repos with few stars heavily.
This is basically just a continuation of that.

In the diff, you'll mostly find deletions. This PR does not do much other than removing the code paths that were only enabled in dotcom mode in the repo syncer, and then removes code that became unused as a result of that.

Test plan

Ran a dotcom mode instance locally, it did not behave differently than a regular instance wrt. repo cloning.
We will need to verify during the rollout that we're not suddenly hitting code paths that don't scale to the dotcom size.

Changelog

Dotcom no longer clones repos on demand.