Avoid using comments_count counter_cache at Investment

As Budget::Investment has two relationships over commentable polymorphic
relationship, the counter_cache is counting the sum of both comments and
valuations.

We don't show valuations count anywhere, only the (public) comments so
we just use comments.count in this case

app/models/budget/investment.rb

@@ -89,6 +89,10 @@ class Budget
     before_validation :set_responsible_name
     before_validation :set_denormalized_ids
 
+    def comments_count
+      comments.count
+    end
+
     def url
       budget_investment_path(budget, self)
     end