remove Singleton from io/async deps

Summary: unused, caused circular dep issues

Test Plan: compiles

Reviewed By: mzlee@fb.com

Subscribers: subodh, folly-diffs@, yfeldblum, chalfant

FB internal diff: D2135780

Signature: t1:2135780:1433786607:015cfa9bb14472f55f80467f2cfeb7426be9f7ea

folly/io/async/AsyncSocket.cpp

@@ -18,7 +18,6 @@
 
 #include <folly/io/async/EventBase.h>
 #include <folly/io/async/EventHandler.h>
-#include <folly/Singleton.h>
 #include <folly/SocketAddress.h>
 #include <folly/io/IOBuf.h>