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.

This also removes the byHash() "pool" special case hack which ends up unused after the fix.