devmapper: Use a common delete function for all device deletion operation

Right now setupBaseImage() uses deleteDevice() to delete uninitialized
base image while rest of the code uses DeleteDevice(). Change it and
use a common function everywhere for the sake of uniformity.

I can't see what harm can be done by doing little extra locking done
by DeleteDevice().

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -516,7 +516,7 @@ func (devices *DeviceSet) setupBaseImage() error {
 
 	if oldInfo != nil && !oldInfo.Initialized {
 		log.Debugf("Removing uninitialized base image")
-		if err := devices.deleteDevice(oldInfo); err != nil {
+		if err := devices.DeleteDevice(""); err != nil {
 			return err
 		}
 	}