Remove root_name class method from Serializer, as it is used nowhere.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 Breaking changes:
 
+- [#1131](https://github.com/rails-api/active_model_serializers/pull/1131) Remove Serializer#root_name (@beauby)
 - [#1138](https://github.com/rails-api/active_model_serializers/pull/1138) Introduce Adapter::Base (@bf4)
   * Adapters now inherit Adapter::Base. 'Adapter' is now a module, no longer a class.
     * using a class as a namespace that you also inherit from is complicated and circular at time i.e.

lib/active_model/serializer.rb

@@ -98,10 +98,6 @@ module ActiveModel
       ActiveModel::Serializer::Adapter.lookup(config.adapter)
     end
 
-    def self.root_name
-      name.demodulize.underscore.sub(/_serializer$/, '') if name
-    end
-
     def self.serializers_cache
       @serializers_cache ||= ThreadSafe::Cache.new
     end