remove tmpfs graph dir setup
remove tmpfs graph dir setup

now that garden-linux uses btrfs, it can just use the native btrfs
driver. yay! no more arbitrary limits or vfs fallback.

Pivotal-DataFabric/docker-image-resource,concourse/docker-image-resource,barthy1/docker-image-resource,barthy1/docker-image-resource,concourse/docker-image-resource,SillyMoo/docker-image-resource,SillyMoo/docker-image-resource,ggeorgiev/docker-image-resource,ggeorgiev/docker-image-resource