[IRGen] NFC: Remove outdated comment about number of async parameters

Author: xedin

lib/IRGen/GenThunk.cpp

@@ -164,8 +164,6 @@ void IRGenThunk::prepareArguments() {
 
   // Chop off the async context parameters.
   if (isAsync) {
-    // FIXME: Once we remove async task and executor this should be one not
-    // three.
     unsigned numAsyncContextParams =
         (unsigned)AsyncFunctionArgumentIndex::Context + 1;
     (void)original.claim(numAsyncContextParams);