As discussed under Alma 8:18, we can have either “the city of Ammonihah” or “the city Ammonihah” in the Book of Mormon text. In each case, we follow the earliest textual sources, thus “the city Ammonihah” here in Alma 49:15. In this instance, the 1953 RLDS edition added the of, perhaps unintentionally.
Summary: Maintain in Alma 49:15 the earliest reading, “the city Ammonihah” (that is, without the of ).