The barbarian class does not need to be changed. The races do.
If Cimmerians and Picts don't (normally) have access to heavy armor, then that should be indicated in their racial stats. Give them a racial trait that removes Heavy armor proficiency (if the class they choose has it). In exchange, they get some minor benefit.
That fixes the problem of Cimmerian and Pict characters having proficiency with heavy armor, while still letting those races that should/could have access to heavy armor be proficient with it. Why change the class (which effects every character of that class, some of which should have the proficiency) when you can just change the race (which solves the problem without affecting all other races).
That being said, my copy of the 2nd edition core book (first printing, 2007) doesn't give barbarians heavy armor proficiency, only light and medium. My 1st edition copy of the book doesn't give it to them either.
