NOTE: I forgot about thisthis. Apparently, the following should (presumably) be the way to change these settings, but the actual feature has not been implemented yet.
You can change this using UndoOptions. To go back to version 9 behavior, use this:
SetOptions[EvaluationNotebook[], UndoOptions -> {"MaxLength" -> 1, "RememberEvaluations" -> False}] See the other options in the documentation for finer control.