Cut explicit class template instantiations in futures

Summary: Cut remaining explicit class template instantiations in futures.

Reviewed By: mengz0

Differential Revision: D16366146

fbshipit-source-id: e755188bd4378ca713b2d73b74def76f9ad837a7

folly/futures/Future-inl.h

@@ -2657,6 +2657,4 @@ Future<Unit> Timekeeper::at(std::chrono::time_point<Clock> when) {
   return after(std::chrono::duration_cast<Duration>(when - now));
 }
 
-// Instantiate the most common Future types to save compile time
-extern template class Future<Unit>;
 } // namespace folly

folly/futures/Future.cpp

@@ -19,13 +19,6 @@
 #include <folly/SingletonThreadLocal.h>
 #include <folly/futures/ThreadWheelTimekeeper.h>
 
-namespace folly {
-
-// Instantiate the most common Future types to save compile time
-template class SemiFuture<Unit>;
-template class Future<Unit>;
-} // namespace folly
-
 namespace folly {
 namespace futures {