The `restore-from-backup` script starts by deleting existing data
from `srv/mariadb` before starting mariadb and importing backups.

Before this commit, it would only delete the contents of `srv/mariadb`
and crash if this directory did not exist.

But the mariadb service only reinitialises `srv/mariadb` if it does
not exist when the service starts, and instead crashes if it exists
but is merely empty.

So the `restore-from-backup` script must actually delete the whole
directory and not just it's contents, and it must not crash if the
directory already does not exists.