core: RemoveVm: no image checks for LUN based VMs

Since now ImagesHandler.PerformImagesChecks no longer handles VM-level
validations, it's possible to skip it altogether if the VM has no image
disks (i.e., no disks at all, or just LUN disks) associated with it.

Note: This refactoring is a step in a long series of patches intended to
completely remove PerformImagesChecks.

Change-Id: Ief6ebb5d3d5b23075424acc92a616f2d7e338170
Signed-off-by: Allon Mureinik <amureini@redhat.com>