Remove legacy notice for FireAndForgetResponder

Summary:
Quoting lblasa:
History trip:
- It was used at the beginning of times with RSocket.
- RSocket introduced an RPC mechanism so a new responder was introduced which used it, thus making this responder legacy.
- WebSockets is introduced which doesn't have an RPC mechanism by default so this responder became the new non-legacy responder

Reviewed By: lblasa

Differential Revision: D37994288

fbshipit-source-id: 2458aed129058a5bfaafd607e73c52867d9713cf

xplat/Flipper/FireAndForgetBasedFlipperResponder.h

@@ -14,13 +14,6 @@
 namespace facebook {
 namespace flipper {
 
-/* Responder for responding to legacy flipper applications.
-   Originally, flipper desktop used fireAndForget for all messages, so calling
-   the SDK would send a fire and forget message, to which the SDK would respond
-   with another one, with an id field that flipper uses to map it to the
-   original request. This Responder should be used when such requests are
-   received.
- */
 class FireAndForgetBasedFlipperResponder : public FlipperResponder {
  public:
   FireAndForgetBasedFlipperResponder(