Remove Issuable#has_multiple_assignees?

Simply because it's no longer used.

app/models/concerns/issuable.rb

@@ -109,10 +109,6 @@ module Issuable
     def allows_multiple_assignees?
       false
     end
-
-    def has_multiple_assignees?
-      assignees.count > 1
-    end
   end
 
   module ClassMethods