Remove fallback to global credentials in batches

Created by: eseliger

This PR completes the deprecation of external service tokens to be used with batch changes. From now on, all ChangesetSources returned from Sourcer methods are ensured to be properly authenticated with either a site credential or user credential, where applicable. This will also make sure that we will never accidentally use other tokens. That also means that we don't need so strict constraints on the external service anymore, so we can simply read the repo.Sources.CloneURL field and don't have to check if an external service has a token configured. Overall, the logic got much simpler and I was able to eliminate many now redundant tests, as they all just use the already tested sourcer methods now.

Closes https://github.com/sourcegraph/sourcegraph/issues/25394

Test plan

Verified locally that things still work, also adjusted the test suite accordingly.