Add to bulk importer the ability to delete all references from a collection

[paynejd]

This single request deletes all references in a collection:

DELETE /orgs/KenyaMOH/collections/KenyaEMR/references/
{
    "references": ["*"]
}

Bulk importer could do this:

{"__method": "DELETE", "owner": "KenyaMOH", "owner_type": "Organization", "collection": "KenyaEMR", "references": ["*"]}