Simplify managing investment votes

If I'm right, the `investment_votes` instance variable only exists to
avoid several database queries to get whether the current user has
supported each of the investments.

However, that doesn't make much sense when only one investment is shown.
In this case, the number of queries stays the same, and so we can
simplify the code by rendering the component with an optional parameter.

app/controllers/management/budgets/investments_controller.rb

@@ -37,12 +37,10 @@ class Management::Budgets::InvestmentsController < Management::BaseController
   end
 
   def show
-    load_investment_votes(@investment)
   end
 
   def vote
     @investment.register_selection(managed_user)
-    load_investment_votes(@investment)
     respond_to do |format|
       format.html { redirect_to management_budget_investments_path(heading_id: @investment.heading.id) }
       format.js