use taxonomy for supplier id

backend/app/plugins/wiaas/includes/class-wiaas-product.php

@@ -72,7 +72,7 @@ class Wiaas_Product {
 	}
 
 	public static function get_supplier_id($product_id){
-		return get_field('supplier', $product_id) ?: NULL;
+		return wp_get_post_terms($product_id, 'supplier', array('fields' => 'ids'))[0];
 	}
 }