df: remove legacy code skipping "rootfs" monitoring

3512bb1 added code to skip duplicate reporting of `rootfs` mounted on `/`.
f9c1c5b and f0398d0 added generic code to skip any volume mounted twice.

Depending on the order of the entries in `/etc/mtab`, reporting for `/` was
entirely skipped.

This patch basically reverts the first, non-generic patch, as it's
superseded by the 2 others.

Fixes #1402

Signed-off-by: Florian Forster <octo@collectd.org>