remove a NPAPI plugin leftover

Change-Id: I4284ed8e59477e508cec3f02a6d1bf2aa3978dd8
Reviewed-on: https://gerrit.libreoffice.org/44177Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>

configure.ac

@@ -9065,12 +9065,6 @@ else
 fi
 
 if test "$USING_X11" = TRUE; then
-    dnl ===================================================================
-    dnl Check for Composite.h for Mozilla plugin
-    dnl ===================================================================
-    AC_CHECK_HEADERS(X11/Composite.h,[],[AC_MSG_ERROR([Xt headers not found])],
-     [#include <X11/Intrinsic.h>])
-
     dnl ===================================================================
     dnl Check for extension headers
     dnl ===================================================================