Do not use call when calling feature style function

Following up on the discussion in #3010 this PR suggests not using `call` when calling the style function bound to a feature.

So with this the user needs to explicitly pre-binds his function to the feature to get the same behavior as before:

```js
var feature = new ol.Feature(geom);
feature.setStyle(function(resolution) {
  // "this" is a reference to the feature
  // …
}.bind(feature));
```

@tschaub, @ahocevar what do you think?