It was not so much that Amy simply forgot about the Daleks.
The Cracks in Time had swallowed up the events of the Dalek invasion. So, Amy didn't forget about it, because it never happened. It was erased from history. Only the Doctor and some of his companions who were there at the time would remember the events. Because for everyone else they literally never happened at all.
Whether or not everyone truly forgot/never knew it happened is unknown because "The Big Bang" establishes that the Doctor restored time to its proper order, so all "swallowed" time was returned (i.e. Amy's parents, human Rory, etc.) This is proven in part by the fact Sarah Jane Smith, in "Death of the Doctor" (Sarah Jane Adventures story), an episode set after the events of "The Big Bang" (Amy and Rory are said to be on their honeymoon) is able to recall the events of "The Stolen Earth".