No, the AMA has made no such stance.
I have combed the AMA's Press Releases from the last several months, with no mention of anything related to the validity of AD/HD.
Historically, as outlined in user15736's answer, the AMA has been for AD/HD as a valid diagnoses, and it would be highly unusual for them to release an opposing stance. So unusual would this be, I contend, that it would surely have made it's way to various media outlets and websites, and it would this be easy to find with a quick Google search.
I also looked for information from the American Psychological Association (APA), which may have been confused with the AMA. However, searching their resources shows not only that they still support AD/HD, but already have future materials planned on the matter, such as a children's book, Learning to Be Kind and Understand Differences: Empathy Skills for Kids With AD/HD, due to be released in October of this year (3 months from now).
In summary, the person who gave you this information has likely misread or misinterpreted other information, or attributed information they found to an incorrect source. It would be best to ask them for the exact source they received the information from.