Remove instance level attr_readonly setting was deprecated.

activerecord/lib/active_record/readonly_attributes.rb

@@ -20,11 +20,5 @@ def readonly_attributes
         self._attr_readonly
       end
     end
-
-    def _attr_readonly
-      message = "Instance level _attr_readonly method is deprecated, please use class level method."
-      ActiveSupport::Deprecation.warn message
-      defined?(@_attr_readonly) ? @_attr_readonly : self.class._attr_readonly
-    end
   end
 end

activerecord/test/cases/base_test.rb

@@ -581,12 +581,6 @@ def test_readonly_attributes
     assert_equal "changed", post.body
   end
 
-  def test_attr_readonly_is_class_level_setting
-    post = ReadonlyTitlePost.new
-    assert_raise(NoMethodError) { post._attr_readonly = [:title] }
-    assert_deprecated { post._attr_readonly }
-  end
-
   def test_non_valid_identifier_column_name
     weird = Weird.create('a$b' => 'value')
     weird.reload