Remove to_ary from RackBody

It was added in 66f8997 to be compatible with Rack::ContentLength.
However, to_ary was removed from Rack::Response in 2.1.0, and
Rack::ContentLength stopped checking for response bodies to define
to_ary in 2.2.0. In addition, Rack 3 will eventually require response
bodies that define to_ary to have a proper return value.

Since the minimum supported Rack version is already 2.2.0, to_ary can
be safely removed now.

actionpack/lib/action_dispatch/http/response.rb

@@ -502,10 +502,6 @@ def respond_to?(method, include_private = false)
       def to_path
         @response.stream.to_path
       end
-
-      def to_ary
-        nil
-      end
     end
 
     def handle_no_content!