RFC: Getter CP readOnly by default

I'd remove this line. I also think you're missing the original motivation, which was to encapsulate a pattern that was **extremely** common before we added this behavior, which was something like:

```js
Ember.Object.extend({
  version: function(key, value) {
    if (arguments.length >= 2) {
      this._version = value;
      return value;
    } else if (this._version) {
      return this._version;
    } else {
      return 1;
    }
  }.property()
})
```

This kind of lazy default was actually reasonably common, and I don't think it's a use-case we should pretend is completely obscure and absurd.