BEIRUT — Syrian government forces used warplanes, helicopters and artillery on Sunday to pound districts of the capital held by the Islamic State group, in a bid to enforce an evacuation deal reached with the militants earlier in the week. Two Palestinian refugees, a father and a son, were killed during the fighting at the Yarmouk refugee camp in Damascus, according to the United Nation’s Palestinian refugee agency, which added that thousands of homes have been destroyed in four days of fighting. Hundreds of IS militants hold parts of the Yarmouk camp and nearby area of Hajar al-Aswad in southern Damascus.