After the import has finished, we flush (delete) the InternalId records
related to the project at hand. This means we're starting over with
tracking correct internal id values, a new record will be created
automatically when the next internal id is generated.

The GitHub importer assigns iid values by using supplied values from
GitHub. We skip tracking internal id values during the import in favor
of higher concurrency. Deleting any InternalId records after the import
has finished is an extra measure to guarantee consistency.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54270.