Remove needless factory

When you get the form from `FormElementManager`, it automatically injects the updated instance of `InputFilterPluginManager` for you.

So you can only provide the input filter specification, and `Zend\Form` will create the input filter for you through the updated `InputFilterPluginManager` (which get validators from the updated `ValidatorPluginManager`).

Ok, I know, it's a bit hard to understand :smile:.

Also I didn't tested it, but it should work. Lemme know if you have some trouble ;)