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

openr/nl/NetlinkProtocolSocket.cpp

@@ -487,7 +487,7 @@ NetlinkProtocolSocket::addRoute(const openr::fbnl::Route& route) {
       // NOTE: We ignore the error for the deleteRoute
       deleteRoute(route);
     }
-    FOLLY_FALLTHROUGH;
+    [[fallthrough]];
   case AF_INET:
     status = rtmMsg->addRoute(route);
     break;