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.

lib/active_model/serializer.rb

@@ -31,7 +31,6 @@ module ActiveModel
     class << self
       attr_accessor :_attributes
       attr_accessor :_attributes_keys
-      attr_accessor :_urls
       attr_accessor :_cache
       attr_accessor :_fragmented
       attr_accessor :_cache_key
@@ -44,7 +43,6 @@ module ActiveModel
     def self.inherited(base)
       base._attributes = self._attributes.try(:dup) || []
       base._attributes_keys = self._attributes_keys.try(:dup) || {}
-      base._urls = []
       base._cache_digest = digest_caller_file(caller.first)
       super
     end
@@ -86,14 +84,6 @@ module ActiveModel
       @_cache_options = (options.empty?) ? nil : options
     end
 
-    def self.url(attr)
-      @_urls.push attr
-    end
-
-    def self.urls(*attrs)
-      @_urls.concat attrs
-    end
-
     def self.serializer_for(resource, options = {})
       if resource.respond_to?(:serializer_class)
         resource.serializer_class