Removed unused `IssuablesHelper#issuable_state_scope`

Its last usage has been removed at 260c8da0.

app/helpers/issuables_helper.rb

@@ -305,14 +305,6 @@ module IssuablesHelper
     cookies[:collapsed_gutter] == 'true'
   end
-  def issuable_state_scope(issuable)
-    if issuable.respond_to?(:merged?) && issuable.merged?
-      :merged
-    else
-      issuable.open? ? :opened : :closed
-    end
-  end
-
   def issuable_templates(issuable)
     @issuable_templates ||=
       case issuable