Bugfix: Hide `stats` progress bar in JSON mode

Since restic 0.19.0, the `stats` command shows a progress bar. This
progress bar was unintentionally displayed also when using the `--json`
option, mixing regular text output with JSON. This is now fixed.

https://github.com/restic/restic/issues/21866
https://github.com/restic/restic/pull/21871
