Revert accidental Vulkan diagnostic commit.

This reverts commit 59a6758fce.

samples/hellopbr.cpp

@@ -52,7 +52,6 @@ int main(int argc, char** argv) {
     Config config;
     config.title = "hellopbr";
     config.iblDirectory = FilamentApp::getRootAssetsPath() + IBL_FOLDER;
-    config.backend = Backend::VULKAN;
 
     App app;
     auto setup = [config, &app](Engine* engine, View* view, Scene* scene) {