KERBALA, Iraq, April 7 (Reuters) - Polish troops killed the head of militant Iraqi cleric Moqtada al-Sadr's office in Kerbala during clashes in the holy Shi'ite city on Wednesday, Iraqi police said. Police spokesman Rahman Mashawi told Reuters Murtada al-Mussawi, who ran Sadr's Kerbala office, was killed in fighting with Polish troops in the centre of town. There was no immediate comment from Polish forces, who head a multinational division in the area. Sadr's militias have been battling occupying forces across south and central Iraq since Sunday. Sadr has rallied supporters with calls for an end to the occupation. The...