Trying to limit access to custom meta box without success

As @totels says, just apply the condition when you add the box, rather than trying to remove it later (FYI, it’s not working because you call the removal on hooks that fire earlier!)

add_action( 'add_meta_boxes', 'assistant_editor_box' );
function assistant_editor_box() {
    if ( current_user_can( 'edit_others_posts' ) )
        add_meta_box(
            'assistant_editor_box',
            ...
        )
}