Clean up import/export files on Geo secondaries

They are already cleaned up on the primary by ImportExportProjectCleanupWorker.

From comment https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6567#note_88894801:

Yes, these files won't be deleted on the Geo secondary. So we either need to create a Geo event as you suggested, or ensure that the Upload records are destroyed when the files are cleaned up.

It sounds like we need to create a separate issue to delete the import/export archives. I think it's better to keep around data than not have it all, but I wonder if there are implications for leaving these old archives around.

cc @stanhu @jameslopez