codeintel: Remove upload files for orphaned commits

Created by: efritz

See https://github.com/sourcegraph/sourcegraph/issues/12588 for a description of how uploads can be orphaned.

We should add a pass to the bundle manager janitor that looks for (sufficiently old) upload records that do not have a corresponding entry in the lsif_nearest_uploads table and removes them, as these uploads are by definition unreachable from the query path.