WASHINGTON — The Obama administration plans to announce on Thursday that it will scrap former President George W. Bush’s planned missile defense system in Poland and the Czech Republic and instead deploy a reconfigured system aimed more at intercepting shorter-range Iranian missiles, according to people familiar with the plans. President Obama decided not to deploy a sophisticated radar system in the Czech Republic or 10 ground-based interceptors in Poland, as Mr. Bush had planned. Instead, the new system his administration is developing would deploy smaller SM-3 missiles, at first aboard ships and later probably either in southern Europe or Turkey,...