Remove batch functionality from sync-changeset subsystem

Created by: mrnugget

@eseliger and I discovered that we never load/sync more than one changeset at once. The batch functionality is a leftover from a year ago, when we synced multiple changesets in the same call.

So after having to deal with "batch functionality and not-found-errors" in https://github.com/sourcegraph/sourcegraph/pull/14771 I did a little spike to see what it would take to remove this code.

Here we go.

Reviewers: what do you think?