Remove unneeded iostream include

Summary:
Including `iostream` in a header causes anything that uses this header to gain
an extra static initializer for an `std::ios_base::Init` object.
If there are N translation units in a library that use the header, that's N
more static initializers that need to run.

Reviewed By: yfeldblum

Differential Revision: D4191798

fbshipit-source-id: 65b47401b9177177999cff65a47ac8dffa0e0546

folly/experimental/LockFreeRingBuffer.h

@@ -18,7 +18,6 @@
 
 #include <atomic>
 #include <boost/noncopyable.hpp>
-#include <iostream>
 #include <cmath>
 #include <memory>
 #include <string.h>