Simplify investment params filters in valuation

We weren't allowing the `budget_id` parameter and then we were adding it
manually. We were also allowing other parameters that aren't used in the
valuation section.

So we're allowing budget and heading, which are the only parameter we're
offering filters for in the user interface. Note the `budget_id`
parameter doesn't seem to make sense because we're already inside a
`@budget.investments` statement, but the `budget_id` parameter is
required by the `scoped_filter` method.

app/controllers/valuation/budget_investments_controller.rb

@@ -18,7 +18,7 @@ class Valuation::BudgetInvestmentsController < Valuation::BaseController
     @heading_filters = heading_filters
     @investments = if current_user.valuator? && @budget.present?
                      @budget.investments.visible_to_valuator(current_user.valuator)
-                            .scoped_filter(filtered_params, @current_filter)
+                            .scoped_filter(params.permit(:budget_id, :heading_id), @current_filter)
                             .order(cached_votes_up: :desc)
                             .page(params[:page])
                    else
@@ -93,10 +93,6 @@ class Valuation::BudgetInvestmentsController < Valuation::BaseController
       end
     end
 
-    def filtered_params
-      Budget::Investment.filter_params(params).to_h.except(:valuator_id).merge(budget_id: @budget.id)
-    end
-
     def valuation_params
       params.require(:budget_investment).permit(allowed_params)
     end