Unstickit simplification

Minor simplification to `unstickit`

Can also be written like below to avoid the slight behaviour change

```js
var models = [], destroyFns = [];
_.each(this._modelBindings, function(binding, i, bindings) {
        if (model && binding.model !== model) return;
        if (bindingSelector && binding.config.selector != bindingSelector) return;

        binding.model.off(binding.event, binding.fn);
        destroyFns.push(binding.config._destroy);
        models.push(binding.model);
        delete bindings[i];
});
```