A slain Brooklyn slumlord's shady business practices helped sink a bank that counted President Obama as a customer and ex-Chicago Bears quarterback Jim McMahon as a member of its board, The Post has learned. Two unpaid loans to murder victim Menachem.......professional hit by one of his many unpaid creditors, source said. Law-enforcement sources said Sunday that investigators are poring over a smartphone found Friday attached to the bottom of StarkÂ’s car. They believe it might have been used to track his movements ahead of his deadly Jan. 2 abduction. Cops are trying to trace the phoneÂ’s owner, as well as...