Delete old elasticsearch backups on restore

`ghe-restore` will sometimes run `ghe-import-es-indices` to restore the elasticsearch backup. However, before it restores the elasticsearch indices, it first takes a backup of the existing elasticsearch indices to the `/home/admin` directory. These backup directories has the tendency to accumulate after multiple runs of the `ghe-restore` script. To prevent this from happening, the changes in this PR tries to delete previous backups generated by the `ghe-import-es-indices`, before running it again.