docs/buildroot.html: remove reference to unused BUILDROOT_USE_XWINDOWS

    <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