Hmm, a lazy workaround—as long as you’re not running PHP < 5.3.0—might be to use a namespace in your mu-plugins
file.
Something like this might do the trick:
<?php
namespace Arglebargle;
\add_action( 'wp_dashboard_setup', 'Arglebarle\remove_x_widget' );
function remove_x_widget() {
// If the meta box has already been removed, it shouldn't be an
// issue to 'remove' it again.
\remove_meta_box( 'x_dashboard_widget', 'dashboard', 'normal' );
}
This code is untested. I hope it provides a good starting point.