cleanup the history of "errorneously" created assets.json etc files on S3 bucket

https://github.com/dandi/dandi-api/issues/537 was addressed by https://github.com/dandi/dandi-api/pull/538 but all those prior versions are still there:

```
> datalad ls -a s3://dandiarchive/dandisets/000108/draft/
Connecting to bucket: dandiarchive
[INFO ] S3 session: Connecting to the bucket dandiarchive anonymously
Bucket info:
Versioning: {'Versioning': 'Enabled'}
Website: dandiarchive.s3-website-us-east-1.amazonaws.com
ACL:
dandisets/000108/draft/assets.jsonld 2021-10-13T13:50:13.000Z 3797418
dandisets/000108/draft/assets.jsonld 2021-10-13T13:50:13.000Z DeleteMarker
dandisets/000108/draft/assets.jsonld 2021-10-08T02:19:21.000Z 3795822
dandisets/000108/draft/assets.jsonld 2021-10-08T02:19:21.000Z DeleteMarker
dandisets/000108/draft/assets.jsonld 2021-10-08T02:11:19.000Z 3794468
... many more
```

saw them while checking out https://github.com/dandi/dandi-api/issues/580