Remove unnecessary access to the old swapchain just for getting image count (#1227)

The image count is only used to teardown per-frame resources
That's already a vector, so we can simply iterate over that

Author: SaschaWillems

samples/api/hello_triangle/hello_triangle.cpp

@@ -539,12 +539,9 @@ void HelloTriangle::init_swapchain()
 			vkDestroyImageView(context.device, image_view, nullptr);
 		}
 
-		uint32_t image_count;
-		VK_CHECK(vkGetSwapchainImagesKHR(context.device, old_swapchain, &image_count, nullptr));
-
-		for (size_t i = 0; i < image_count; i++)
+		for (auto &per_frame : context.per_frame)
 		{
-			teardown_per_frame(context.per_frame[i]);
+			teardown_per_frame(per_frame);
 		}
 
 		context.swapchain_image_views.clear();