Leonard Garment, special assistant to the President, at press conference says is going back to his original government post. Finished one year as President Nixon's top man on Watergate. Talks about his White House service and Watergate as time of frustration. Says Nixon won't be impeached. Garment: Watergate has affected the social climate, this is a serious problem, the president reconizes the difficultes it has created, and problems encountered carrying out programs.