Remove investment supports query optimizations

In the previous commit I mentioned:

> 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.

Now let's discuss the case when there are several investments, like in
the investments index:

* There are 10 investments per page by default
* Each query takes less than a millisecond
* We still make a query per investment to check whether the current user
  voted in a different group
* AFAIK, there have been no performance tests showing these
  optimizations make the request to the investments index significantly
  faster
* These optimizations make the code way more complex than it is without
  them

Considering all these points, I'm removing the optimizations. I'm fine
with adding `includes` calls to preload records and avoid N+1 queries
even if there are no performance tests showing they make the application
faster because the effect on the code complexity is negligible. But
that's not the case here.

Note we're using `defined?` instead of the `||=` operator because the
`||=` operator will not serve its purpose when the result of the
operation returns `false`.

app/controllers/management/budgets/investments_controller.rb

@@ -15,7 +15,6 @@ class Management::Budgets::InvestmentsController < Management::BaseController
 
   def index
     @investments = @investments.apply_filters_and_search(@budget, params).page(params[:page])
-    load_investment_votes(@investments)
   end
 
   def new
@@ -49,15 +48,10 @@ class Management::Budgets::InvestmentsController < Management::BaseController
 
   def print
     @investments = @investments.apply_filters_and_search(@budget, params).order(cached_votes_up: :desc).for_render.limit(15)
-    load_investment_votes(@investments)
   end
 
   private
 
-    def load_investment_votes(investments)
-      @investment_votes = managed_user ? managed_user.budget_investment_votes(investments) : {}
-    end
-
     def investment_params
       attributes = [:external_url, :heading_id, :tag_list, :organization_name, :location,
                     image_attributes: image_attributes,