core: cleanup related to removal of memory volumes

1. Move VmCommand#shouldRemoveMemoryVolumes method to
RemoveSnapshotCommand as it is used only by this class and rename it to
isMemoryVolumeRemoveable

2. Remove RemoveVmCommand#shouldRemoveMemoryVolumes method which is not
in use

Change-Id: I55df977799f79cf91defc111d32b7663a305e60b
Signed-off-by: Arik Hadas <ahadas@redhat.com>