devmapper: Fix shutdown warnings

Shutdown contains debug warnings like:

[debug] deviceset.go:699 [deviceset docker-0:33-17945897] waitRemove(/dev/mapper/docker-0:33-17945897-pool)
[debug] deviceset.go:380 libdevmapper(3): libdm-common.c:552 (-1) Device /dev/mapper/docker-0:33-17945897-pool not found

This is because shutdown is using removeDeviceAndWait() to remove the pool device and the
wait part fails because the pool is gone.

We fix this by adding a pool specific removal function which avoids all the trickiness of the
normal remove.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Remove byHash hack

We no longer pass "pool" anywhere that uses byHash() per the last
commit, so we can now remove this hack.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4062 from alexlarsson/clean-shutdown

devmapper: Fix shutdown warnings