use "attr_reader" instead of "attr_accessor" for CustomFieldValue#value

CustomFieldValue#value= is already defined in this class.

git-svn-id: http://svn.redmine.org/redmine/trunk@18615 e93f8b46-1217-0410-a6f0-8f06a7374b81

.rubocop_todo.yml

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

app/models/custom_field_value.rb

@@ -18,7 +18,8 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
 class CustomFieldValue
-  attr_accessor :custom_field, :customized, :value, :value_was
+  attr_accessor :custom_field, :customized, :value_was
+  attr_reader   :value
 
   def initialize(attributes={})
     attributes.each do |name, v|