Use OrderContents in BulkLineItemsController

We should move towards *all* operations on an order's line items being done exclusively through this service.

app/controllers/admin/bulk_line_items_controller.rb

@@ -49,7 +49,7 @@ module Admin
       load_line_item
       authorize! :update, order
 
-      @line_item.destroy
+      order.contents.remove(@line_item.variant, @line_item.quantity)
       render body: nil, status: :no_content # No Content, does not trigger ng resource auto-update
     end
 

spec/controllers/admin/bulk_line_items_controller_spec.rb

@@ -378,9 +378,6 @@ describe Admin::BulkLineItemsController, type: :controller do
         expect(order.included_tax_total).to eq 1.22
         expect(order.payment_state).to eq "paid"
 
-        expect(order).to receive(:update_order!).at_least(:once).and_call_original
-        expect(order).to receive(:create_tax_charge!).at_least(:once).and_call_original
-
         spree_delete :destroy, params
         order.reload