GMAC Mortgage, one of the nation’s leading lenders, said yesterday that it will end most of its mortgage business in Massachusetts, a day after Attorney General Martha Coakley sued the company and four other major financial firms over their role in the foreclosure crisis. A spokeswoman for GMAC’s parent, Ally Financial Inc., said GMAC will no longer buy loans that originated with other lenders and mortgage brokers in Massachusetts, but will honor all commitments made through Monday.