By David Shepardson, Valerie Insinna and Tim Hepher WASHINGTON/DUBLIN (Reuters) – Boeing said on Monday it was withdrawing a request for a key safety exemption that could have allowed U.S. regulators to speed up certification of its coming 737 MAX 7, in the latest fallout from the planemaker’s ongoing crisis. Lawmakers had been pressuring the planemaker to withdraw the petition following a harrowing mid-air cabin blowout on Jan. 5 that has exposed numerous safety and quality control concerns at one of the world’s two major jetmakers. No one died in the blowout that left passengers staring at o…