Desktop samples now use default backend.

There is nothing specific to Vulkan in these tests, I meant to remove
the backend override when I removed the `vk_` prefix last week.

samples/depthtesting.cpp

@@ -65,7 +65,6 @@ static constexpr uint16_t TRIANGLE_INDICES[3] = { 0, 1, 2 };
 int main(int argc, char** argv) {
     Config config;
     config.title = "depthtesting";
-    config.backend = Engine::Backend::VULKAN;
 
     App app;
     auto setup = [&app](Engine* engine, View* view, Scene* scene) {