wordpress-filter-uncategorised.php

<?php

add_filter( 'get_the_categories', 'adt_remove_uncategorised_post_category', 20, 2 );

function adt_remove_uncategorised_post_category( $categories, $id ) {
    return array_filter($categories, function($a) {
        return 'uncategorised' === $a->slug ? false : $a;
    });
}