[libxml2] configure.ac: remove useless AC_SUBST

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 7a4037bea2fa4f29af07e0328192c2e224add257
Author: David Seifert <soap gentoo org>
Date:   Wed Mar 30 16:51:12 2022 +0200

    configure.ac: remove useless AC_SUBST
    
    * CPPFLAGS/LDFLAGS are always substituted

 configure.ac | 2 --
 1 file changed, 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9e2a7504..4a901999 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1455,9 +1455,7 @@ else
     echo Disabling code coverage for GCC
 fi
 
-AC_SUBST(CPPFLAGS)
 AC_SUBST(EXTRA_CFLAGS)
-AC_SUBST(LDFLAGS)
 AC_SUBST(XML_CFLAGS)
 
 AC_SUBST(XML_LIBDIR)