House Majority Whip, Charles A. Murphy, facing certain defeat in any effort to hold his office, resigned his leadership position at a Democratic caucus this morning, saying he wanted to avoid putting his colleagues through a vote on his removal that was being engineered by House Speaker Robert A. DeLeo. DeLeo filled Murphy’s whip position with veteran state representative Byron Rushing, a South End Democrat, who has been serving as a Floor Division chair, a post that has few duties.