By Roberta Rampton and Stephen Brown BERLIN (Reuters) - U.S. President Barack Obama used a speech in Berlin on Wednesday to call on Russia to revive the push for a world without nuclear weapons, offering to cut deployed nuclear arsenals by a third, but Moscow immediately poured scorn on his proposal. Speaking in Berlin where U.S. Presidents John F. Kennedy and Ronald Reagan gave rousing Cold-War speeches, Obama urged Russia to help build on the "New START" treaty that requires Moscow and Washington to cut stockpiles of deployed nuclear weapons to 1,550 each by 2018. ...