Remove redundand assignment

I initially added this in order to keep the code more error-prone to
following additions, but it seems it's still frowned upon.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_domain.c

@@ -831,7 +831,6 @@ qemuDomainObjPrivateXMLParse(xmlXPathContextPtr ctxt,
             goto error;
     }
     virObjectUnref(caps);
-    caps = NULL;
     VIR_FREE(tmp);
 
     return 0;