Under mounting political, legal and financial pressure, Sinclair Broadcast Group Inc. yesterday backed away from its plan to carry a film attacking John F. Kerry's Vietnam War record, saying it would air only portions of the movie in an hour-long special scheduled for Friday. "The experience of preparing to air this news special has been trying for many of those involved," Sinclair chief executive David D. Smith said in a statement. "The company and many of its executives have endured personal attacks of the vilest nature, as well as calls on our advertisers and our viewers to boycott our stations...