Remove unused type alias

Summary: The ValueType alias in the multiplex function is not used and caused a build failure when I tried to use it, possibly because we have some extra warnings enabled in our code.

Reviewed By: lewissbaker

Differential Revision: D21685780

fbshipit-source-id: 3235c6444a623d5f484f0849679b1497f9456fa9

folly/experimental/coro/Multiplex-inl.h

@@ -35,7 +35,6 @@ multiplex(
     SelectIdFn&& selectId) {
   using EventType = CallbackRecord<Reference>;
   using ReferenceType = Enumerated<KeyType, EventType>;
-  using ValueType = Enumerated<KeyType, CallbackRecord<Value>>;
 
   return merge(
       std::move(exec),