Remove unnecessary safe navigation operator

The heading is used with `find_by_slug_or_id`, which raises an exception
if it isn't found, so executing `@heading.group` after it does not need
the safe navigation operator.

app/controllers/budgets/investments_controller.rb

@@ -148,7 +148,7 @@ module Budgets
       def load_heading
         if params[:heading_id].present?
           @heading = @budget.headings.find_by_slug_or_id! params[:heading_id]
-          @assigned_heading = @ballot&.heading_for_group(@heading&.group)
+          @assigned_heading = @ballot&.heading_for_group(@heading.group)
           load_map
         end
       end