repo-updater: Do not try to source deleted external services
Created by: keegancsmith
This PR introduces two changes:
- Do not block deletion on triggering repo-updater syncs. We don't need to give the user any feedback from repo-updater, so no need to wait for the trigger to happen.
- Do not try source deleted external services. We source for validation purposes, but no point validating a deleted external service.
The second point in particular lead to the reported bug report where we would present a warning that we failed to sync when deleting an external service. However, this was expected since the external service upstream code host was down, which is why the service was deleted.
Test plan: unit tests
Fixes https://github.com/sourcegraph/sourcegraph/issues/5617