Minor BugFix : Missing delete Category Multilangual fields

Delete of Multilangual fields was missing, impossible to delete category.

Fixed Error : 
Cannot delete or update a parent row: a foreign key constraint fails (`Dol-3.6`.`llx_categorie_lang`, CONSTRAINT `fk_category_lang_fk_category` FOREIGN KEY (`fk_category`) REFERENCES `llx_categorie` (`rowid`))