Clean up single user mode

Single user mode basically disables WebDAV, OCS and cron execution. Since
we heavily rely on WebDAV and OCS also in the web UI it's basically useless.
An admin only sees a broken interface and can't even change any settings nor
sees any files. Also sharing is not possible.

As this is at least the case since Nextcloud 9 and we haven't received any
reports for this it seems that this feature is not used at all so I removed it.

The encryption commands now rely on the well tested maintenance mode.

Signed-off-by: Morris Jobke <hey@morrisjobke.de>