Remove various unused value assignments

As detected by Coverity (CIDs 60467–60472).

https://bugzilla.gnome.org/show_bug.cgi?id=739220
diff --git a/relaxng.c b/relaxng.c
index 8d88e95..5357b56 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -9859,7 +9859,7 @@
     ctxt->depth++;
     switch (define->type) {
         case XML_RELAXNG_EMPTY:
-            node = xmlRelaxNGSkipIgnored(ctxt, node);
+            xmlRelaxNGSkipIgnored(ctxt, node);
             ret = 0;
             break;
         case XML_RELAXNG_NOT_ALLOWED: