No way to delete a user-role mapping in Rule Based Authorization Plugin

I don't have time to work on this soon, but filing this so it's not lost. I recently typoed the name of a user when assigning a role, and got a useless user-role mapping, and there does not appear to be a command in org.apache.solr.security.AutorizationEditOperation (in my slightly outdated branch_8x working copy at least) for deleting this mapping. Also the ref guide does not document such. We should probably do two things here:

1.) Validate that the user being mapped exists as a user, 400 Bad request if not

2.) Provide a means to remove the mapping entirely (delete-user-role)