DeviceMapper: Succeed immediately when removing non-existant devices

We've seen situations where removal of "ID-init" failed during
container deletion (EBUSY), after removal of "ID" has succeeded. This
caused the container delete operation to fail, and on the next delete
attempt the removal of "ID" failed immediately with "does not exist".

Ideally we should not fail the ID-init removal, but its also non-ideal
to allow a state where the container is half-removed and we cannot
make progress deleting the container. So, we silently ignore not-exist
errors on device removal.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)