Trump does not have the power to repeal the Johnson Act. He can repeal executive actions taken by previous presidents, but not laws passed by Congress.

Once passed, a law can be repealed by Congress or struck down as unconstitutional by the Supreme Court, but there's nothing the executive branch can do to get rid of it.

All a president can do is use his "bully pulpit" to lobby Congress to repeal the law. A more reasonable president would have phrased it in terms more like, "I will ask Congress to repeal...", but that's not 100% about Trump so you'll never hear it.