use "attr_writer" instead of "attr_accessor" for QueryColumn#sortable

QueryColumn#sortable is already defined in this class.

git-svn-id: http://svn.redmine.org/redmine/trunk@18616 e93f8b46-1217-0410-a6f0-8f06a7374b81

.rubocop_todo.yml

@@ -495,7 +495,6 @@ Lint/DeprecatedClassMethods:
 
 Lint/DuplicateMethods:
   Exclude:
-    - 'app/models/query.rb'
     - 'app/models/wiki_page.rb'
 
 Lint/EmptyWhen:

app/models/query.rb

@@ -20,7 +20,8 @@
 require 'redmine/sort_criteria'
 
 class QueryColumn
-  attr_accessor :name, :sortable, :groupable, :totalable, :default_order
+  attr_accessor :name, :groupable, :totalable, :default_order
+  attr_writer   :sortable
   include Redmine::I18n
 
   def initialize(name, options={})