Hide soft deleted prices from admin product view

The `product.prices` relation has been changed in 3f4578a
to make eager loading possible, but this now shows soft deleted
prices in the admin product prices tab.

Hiding them again to restore previous behavior.

backend/app/controllers/spree/admin/prices_controller.rb

@@ -8,7 +8,7 @@ class PricesController < ResourceController
       def index
         params[:q] ||= {}
 
-        @search = @product.prices.accessible_by(current_ability, :index).ransack(params[:q])
+        @search = @product.prices.kept.accessible_by(current_ability, :index).ransack(params[:q])
         @master_prices = @search.result
           .currently_valid
           .for_master

backend/spec/controllers/spree/admin/prices_controller_spec.rb

@@ -11,15 +11,18 @@
     context "when only given a product" do
       let(:product) { create(:product) }
 
+      let!(:deleted_master_price) { create(:price, variant: product.master).tap(&:discard!) }
+
       subject { get :index, params: { product_id: product.slug } }
 
       it { is_expected.to be_successful }
 
       it 'assigns usable instance variables' do
         subject
         expect(assigns(:search)).to be_a(Ransack::Search)
-        expect(assigns(:variant_prices)).to eq(product.prices.for_variant)
-        expect(assigns(:master_prices)).to eq(product.prices.for_master)
+        expect(assigns(:variant_prices)).to be_empty
+        expect(assigns(:master_prices)).to eq(product.prices.kept.for_master)
+        expect(assigns(:master_prices)).to_not include(deleted_master_price)
         expect(assigns(:product)).to eq(product)
       end
     end
@@ -28,16 +31,19 @@
       let(:variant) { create(:variant) }
       let(:product) { variant.product }
 
+      let!(:deleted_variant_price) { create(:price, variant: variant).tap(&:discard!) }
+
       subject { get :index, params: { product_id: product.slug, variant_id: variant.id } }
 
       it { is_expected.to be_successful }
 
       it 'assigns usable instance variables' do
         subject
         expect(assigns(:search)).to be_a(Ransack::Search)
-        expect(assigns(:variant_prices)).to eq(product.prices.for_variant)
+        expect(assigns(:variant_prices)).to eq(product.prices.kept.for_variant)
         expect(assigns(:master_prices)).to eq(product.prices.for_master)
         expect(assigns(:variant_prices)).to include(variant.default_price)
+        expect(assigns(:variant_prices)).to_not include(deleted_variant_price)
         expect(assigns(:product)).to eq(product)
       end
     end