Remove method_added hook doing nothing:

- ### Context

  In rails#54646, I fixed a bug. The patch introduced a behaviour change
  which was caught by sorbet typechecking.

  ------------------------

  ```ruby
  class Current < ActiveSupport::Current
    def foo
    end
  end
  ```

  Previously:

  ```ruby
  Current.method(:foo).source_location => nil
  ```

  After:

  ```ruby
  Current.method(:foo).source_location => [lib/active_support/current_attributes.rb", 191]
  ```

  --------------------

  Basically, before, we were never creating a delegation method
  to the class, and instead relied on `method_missing` entirely.

  ### Problem

  There is no problem per se, but this method_added hook used to do
  nothing. And the previous patch now creates a delegation on the class.

  This is because when a method is added, `respond_to?` on the
  instance will always return `true` since we have just defined it.
  When calling `attribute :foo`, a method on the **singleton class** gets
  created, but doesn't not trigger our method_added hook defined on the
  **class**.

Author: Edouard-chin

activesupport/lib/active_support/current_attributes.rb

@@ -182,14 +182,6 @@ def method_missing(name, ...)
         def respond_to_missing?(name, _)
           instance.respond_to?(name) || super
         end
-
-        def method_added(name)
-          super
-          return if name == :initialize
-          return unless public_method_defined?(name)
-          return if singleton_class.method_defined?(name) || singleton_class.private_method_defined?(name)
-          Delegation.generate(singleton_class, [name], to: :instance, as: self, nilable: false)
-        end
     end
 
     class_attribute :defaults, instance_writer: false, default: {}.freeze