maintenance:mode produces a warning despite --quiet

Steps to reproduce

1. $ php occ maintenance:mode --on --quiet
2. $ php occ maintenance:mode --off --quiet

Expected behaviour

Maintenance mode is enabled and disabled. No output is produced unless a fatal error occurs.

Actual behaviour

$ php occ maintenance:mode --off --quiet
Nextcloud is in maintenance mode - no apps have been loaded

The message is printed regardless of --quiet and --no-warnings.

This seems like an easily reproducible issue, so I'll take the liberty of omitting further details about my setup.

[weeman1337]

I started working on #9949. This issue here may be solved by that.