The legacy render layer system that was available in the Layer Editor in previous versions of Maya has been replaced by the render setup system. The two systems are separate, independent systems and you can only use one or the other during a given Maya session.
To use the legacy render layer system, select Windows > Settings/Preferences > Preferences to open the Preferences window, and navigate to the Rendering category. Select Legacy Render Layers as the Preferred Render Setup system and restart Maya.