shutdown: don't consider umounting of / and /usr failed

shutdown: don't consider umounting of / and /usr failed

In the words of Homer: If you don't try, you can't fail.

This is a revert of 9279749b84cc87c7830280b7895a48bed03c9429.

It used to be necessary to consider the umounting failed to make sure /
and /usr were remounted read-only, but that is no longer necessary as
everything is now remounted read-only anyway.

Moreover, this avoids a warning at shutdown saying a filesystem was not
unmounted. As the umounting of / is never attempted there was no
corresponding warning message saying which fs that failed. This caused some
spurious bug-reports from concerned users.

Cc: Michael Biebl <biebl@debian.org>