Remove method to get votes for budget investments

After commit 0214184b2, this method was only used in two places and was
only useful in one of them. IMHO it isn't worth it add a monkey-patch
for such a minor usage.

config/initializers/vote_extensions.rb

@@ -13,10 +13,6 @@ ActsAsVotable::Vote.class_eval do
           Comment.public_for_api.pluck(:id))
   end
 
-  def self.for_budget_investments(budget_investments = Budget::Investment.all)
-    where(votable_type: "Budget::Investment", votable_id: budget_investments)
-  end
-
   def value
     vote_flag
   end