Remove temporary members from struct _xmlXPathContext

These values are hardcoded now and the struct members, while public,
were recently introduced and never part of an official release.
diff --git a/include/libxml/xpath.h b/include/libxml/xpath.h
index a4303ff..539593f 100644
--- a/include/libxml/xpath.h
+++ b/include/libxml/xpath.h
@@ -359,8 +359,6 @@
     unsigned long opLimit;
     unsigned long opCount;
     int depth;
-    int maxDepth;
-    int maxParserDepth;
 };
 
 /*