migrate from FOLLY_FALLTHROUGH to [[fallthrough]] (assorted)

Summary:
X-link: facebookincubator/velox#8586

The attribute `[[fallthrough]]` is standard C++17. We no longer need to wrap it.

Reviewed By: jbeshay, Orvid

Differential Revision: D53034073

fbshipit-source-id: 9bf155ffa0e6f797cbce8d63d582c70777c1105e

proxygen/lib/http/session/HQByteEventTracker.cpp

@@ -51,7 +51,7 @@ void HQByteEventTracker::onByteEventWrittenToSocket(const ByteEvent& event) {
   const auto& streamOffset = event.getByteOffset();
   switch (event.eventType_) {
     case ByteEvent::FIRST_BYTE:
-      FOLLY_FALLTHROUGH;
+      [[fallthrough]];
     case ByteEvent::LAST_BYTE: {
       // install TX callback
       {