Deprecate and remove backup layouts

Layouts (backup.Locator in code) were a mechanism to slowly and safely convert backups from using the original directory structure that gitlab-rails used to one that supports incremental backups and other features (default branch and sha1/sha256). As of Switch repository backups to use manifests only (gitlab!144677 - merged) backups now use manifest files. These manifest files store all the paths that we previously needed from the layout. So effectively layouts are no longer required.

This issue is to deprecate and eventually remove:

• The server-side config backup.layout.
• The -layout option for gitaly-backup.

Manifest files are always generated no matter which layout is configured (!6391 (merged)). So even if these options were removed it would be possible to restore a backup with a different layout.