qubes/tests: do not undefine libvirt domain twice

Do not access vm.libvirt_domain after it being already removed - this
will redefine it again in libvirt, just to undefine it in a moment.
On the other hand, few lines below there is fallback libvirt cleanup, in
case of proper one not working.

qubes/tests/__init__.py

@@ -523,11 +523,6 @@ class SystemTestsMixin(object):
         except: # pylint: disable=bare-except
             pass
 
-        try:
-            vm.libvirt_domain.undefine()
-        except (AttributeError, libvirt.libvirtError):
-            pass
-
         del app.domains[vm.qid]
         del vm