remove Railtie.generator

This method is no longer needed, he detects the generator method wich has changed from Rails 2.x to 3.0.0. Mongoid has a dependency to activemodel > 4.0, which related in a dependency to Rails > 4.0. Which means that this check is no longer needed, there can't be a Rails < 4.0
Merge pull request #3867 from timoschilling/patch-3

remove Railtie.generator