backup: Remove legacy layouts
Layouts 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.
Since !6391 (merged) manifest files are always generated no matter which layout is configured. So even if these options were removed, it would be possible to restore a backup with a different layout. These manifest files store all the paths that we previously needed from the layout. So effectively layouts are no longer required.
Furthermore, manifest is already sent as the default layout option from rails since gitlab!144677 (merged)
This MR removes the other layout options from the gitaly code and makes the manifest layout default option.
There is room for more refactoring as we no longer need to pass locator around and directly default to manifest but I wanted to keep this MR manageable and will do the further refactoring once we are ready to remove the layout param from the config and backup tooling completely:
- Default to manifest locator in the gitaly codebase (current), making the -layout config no-op.
- Next version, remove usage of -layout flag from the client (rails)
- Next version, remove the -layout option from the gitaly-backup tooling completely
Relates to #6091 (closed)