pkg/devicemapper: cleanup removeDevice differences

Fixes failure on RemoveDevice when host is AUFS, and running
devicemapper test docker-in-docker

https://gist.github.com/tonistiigi/59559cbfb3f2df26b29c

ping @crosbymichael @tonistiigi 

Signed-off-by: Vincent Batts <vbatts@redhat.com>