[ALOY-1602] Avoid some global variables to be only available in parent controller.

Quote from [Github](https://github.com/appcelerator/alloy/pull/859): {quote} I don't understand why it is necessary to remove "$model", "__parentSymbol" or "__itemTemplate" from the "arguments" variable. This is causing issues when passing "$model" for example to a controller which is inheriting from another controller as per Alloy Controller Inheritance documentation (http://docs.appcelerator.com/platform/latest/#!/guide/Alloy_Controllers-section-src-34636384_AlloyControllers-Inheritance). If we delete it from there, only the parent will have "$model" properly set but the child controller won't as it's deleted from "arguments". Another option would be to run "__processArg()" before calling the parent controller but this would mean only the child controller has $model properly set, it will be null for the parent one. In order to reproduce:

index.js

Alloy.createController('child', \{ $model: aBackboneModelObject \});

parent.js

console.log(_.isNull($model)); // always return false ...

child.js

exports.baseController = "parent"; console.log(_.isNull($model)); // always return true {quote}