Disable xmlExp regex code

This is apparently another regex engine that was never used, see commit
81a8ec6.

Makefile.am

@@ -915,6 +915,9 @@ Regexptests: testRegexp$(EXEEXT)
 	      if [ -n "$$log" ] ; then echo $$name result ; echo "$$log" ; fi ; \
 	      rm result.$$name ; \
 	  fi ; fi ; done)
+
+# Disabled for now
+Exptests: testRegexp$(EXEEXT)
 	@echo "## Formal expresssions regression tests"
 	-@(for i in $(srcdir)/test/expr/* ; do \
 	  name=`basename $$i`; \

include/libxml/xmlversion.h.in

@@ -350,8 +350,10 @@ XMLPUBFUN void XMLCALL xmlCheckVersion(int version);
  * LIBXML_EXPR_ENABLED:
  *
  * Whether the formal expressions interfaces are compiled in
+ *
+ * This code is unused and disabled unconditionally for now.
  */
-#if @WITH_SCHEMAS@
+#if 0
 #define LIBXML_EXPR_ENABLED
 #endif