Remove jQuery workaround that's no longer needed

Code in question: https://github.com/knockout/knockout/blob/1f0e337f84f694d4f7eb17c8b90bee06064f4645/src/utils.js#L301

jQuery 1.9 upgrade notes: http://jquery.com/upgrade-guide/1.9/#checkbox-radio-state-in-a-trigger-ed-click-event

jQuery bug ticket: http://bugs.jquery.com/ticket/3827