Change: Deprecate `check --check-unused` and add further checks

Since restic 0.12.0, it is expected to still have unused blobs after running
`prune`. This made the `check --check-unused` rather useless and tended to
confuse users. The options has been deprecated and is now ignored.

`check` now also warns if a repository is using either the legacy S3 layout or
mixed pack files with both tree and data blobs. The latter is known to cause
performance problems.

https://github.com/restic/restic/issues/3295
https://github.com/restic/restic/pull/3730
