Mesa Challenger Quits Board: A rebellious group of Mesa Inc. shareholders backed away from threats to oust Chairman Boone Pickens and the Irving, Tex.-based natural gas company agreed to raise cash by next spring. Mesa agreed to seek asset sales or outside investment by the May, 1996, annual meeting, but the shareholders said they will again challenge Pickens if the company doesn't live up to the agreement. The dissidents' ringleader, David Batchelder, quit the board and is to be replaced by one of his business partners. Under terms of the agreement, Mesa must achieve a property or asset sale of $640 million, an equity infusion of $250 million, a joint venture of $350 million or any combination of those options. Lehman Bros. Inc. also must say the offer is fair.