Altheimer & Gray partners agreed Thursday to dissolve their law partnership no later than Nov. 1. The firm, which slipped from the city's 12th-largest last year to 19th this year, had been laying off attorneys as clients such as United Airlines filed for bankruptcy and the mergers and acquisitions business slowed down. The firm's political business dried up with former Gov. George Ryan--close friend of partner Jeremy Margolis--leaving office. Altheimer & Gray was dealt a setback last year when a federal