Remove SolrBackup Persistence option

[HoustonPutman]

With the support for GCS and S3 backups (#301, #328), we should remove the Persistence options for the SolrBackup.

It's really just a legacy option to help people store things in S3, before the S3Repository was made available. It's janky and doesn't work with the new incremental backups.

I think it would be best to just remove the option all-together.

[gerlowskija]

+1 from me:

I think taking it out makes sense - especially if it was primarily a means of getting backups into S3 (which we now have other options for). The idea of compressing and moving backups is probably someone specific to non-incremental backups too, which are deprecated and set to go away in the future.