By Jonathan Saul, Simon Lewis and Megan Davies LONDON/WASHINGTON/DAVOS, Switzerland (Reuters) -The U.S. on Wednesday returned the Yemen-based Houthi rebels to a list of terrorist groups, as the militants claimed their second attack this week on a U.S. operated vessel in the Red Sea region. Attacks by the Iran-allied Houthi militia on ships in the region since November have slowed trade between Asia and Europe and alarmed major powers – an escalation of the more than three-month-old war between Israel and Palestinian Hamas militants in Gaza. The Houthis say they are acting in solidarity with Pa…