remove __define{G,S}etter__() as this function is deprecated

__defineGetter__() and __defineSetter__() are deprecated. [1]
Object.defineProperty() is an ECMAScript 5.1 Standard and supported by all
major browsers. [2]

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/__defineGetter__
[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty#Specifications