Remove url options

Removing url options because It does not works at all.
Thus, there are others PR at the moment to include url(links) as well.

test/fixtures/poro.rb

@@ -59,8 +59,6 @@ end
 class ProfileSerializer < ActiveModel::Serializer
   attributes :name, :description
 
-  urls :posts, :comments
-
   def arguments_passed_in?
     options[:my_options] == :accessible
   end
@@ -68,8 +66,6 @@ end
 
 class ProfilePreviewSerializer < ActiveModel::Serializer
   attributes :name
-
-  urls :posts, :comments
 end
 
 Post     = Class.new(Model)
@@ -100,7 +96,6 @@ PostSerializer = Class.new(ActiveModel::Serializer) do
   has_many :comments
   belongs_to :blog
   belongs_to :author
-  url :comments
 
   def blog
     Blog.new(id: 999, name: 'Custom blog')