Blizzard gives no reason for the removal of the Achievement and title, and forum posts asking about the matter have so far received no blue responses. On the other hand, Bornakk was quick to address the bug about the Champion of the Frozen Wastes title, which was hotfixed. It's fairly certain that Blizzard removed the Achievement intentionally. Some suspect that there were exploits that allowed players to obtain the Achievement with ease, while others dismissed it as a meaningless title in what is arguably the most imbalanced Arena season to date. With the Arena system being completely overhauled yet again for Season 6, it is possible that we may see the title return in some form or the other.