devmapper: Remove directory when removing devicemapper device

We're currently leaving around lots of empty directories in
/var/lib/docker/devicemapper/mnt/ for removed images and containers.
Fix this by removing the directory when the device is removed.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4055 from alexlarsson/remove-devmapper-dirs

devmapper: Remove directory when removing devicemapper device