docs/buildroot.html: remove reference to unused BUILDROOT_USE_XWINDOWS

docs/buildroot.html

@@ -254,7 +254,6 @@
     <li>BUILDROOT_COPYTO</li>
     <li>BUILDROOT_DL_DIR</li>
     <li>BUILDROOT_LOCAL</li>
-    <li>BUILDROOT_USE_XWINDOWS</li>
     </ul>
 
     <p>An example that uses config files located in the toplevel directory and