Extirpate FOLLY_NODISCARD in assistant/server/nlg/NlgConfigProd.h +5

Summary:
`[[nodiscard]]` is now available in all of our compilers. This diff changes the code to rely on it directly.

Generated with
```
reset && xbgs -l FOLLY_NODISCARD | grep -v Portability.h | grep -v json | grep -v xplat | grep -v ThirdParty/ | grep -v third-party | perl -pe 's/^fbsource.//' | xargs -n 100 perl -pe 's/FOLLY_NODISCARD/[[nodiscard]]/' -i
```

Reviewed By: dtolnay

Differential Revision: D87101274

fbshipit-source-id: 09bf74b289c97f8c406163146cd618edcf328544

Author: r-barnes

cachelib/allocator/Handle.h

@@ -436,7 +436,7 @@ struct ReadHandleImpl {
   //             if the handle becomes/is ready, this returns the
   //             original callback back to the caller to execute.
   //
-  FOLLY_NODISCARD ReadyCallback onReady(ReadyCallback&& callBack) {
+  [[nodiscard]] ReadyCallback onReady(ReadyCallback&& callBack) {
     return (waitContext_) ? waitContext_->onReady(std::move(callBack))
                           : std::move(callBack);
   }