Use Model::deleteRelated() method to remove relationship between two records.
$model = $client->model('Assembly');
$assembly_id = '278ef520-bab3-546f-0e2f-5aab53353bbf';
$product_id = '6d57ea93-65c3-2779-56be-55b9b0525aec';
$result = $model->deleteRelated($assembly_id, 'products', $product_id);
echo json_encode($result, JSON_PRETTY_PRINT), "\n";
Output: