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.

app/models/user.rb

@@ -153,7 +153,7 @@ class User < ApplicationRecord
   end
 
   def voted_in_group?(group)
-    votes.for_budget_investments(Budget::Investment.where(group: group)).exists?
+    votes.where(votable: Budget::Investment.where(group: group)).exists?
   end
 
   def headings_voted_within_group(group)
@@ -161,7 +161,7 @@ class User < ApplicationRecord
   end
 
   def voted_investments
-    Budget::Investment.where(id: votes.for_budget_investments.pluck(:votable_id))
+    Budget::Investment.where(id: votes.where(votable: Budget::Investment.all).pluck(:votable_id))
   end
 
   def administrator?

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