boost: remove iostreams includes

They're unused and produce nasty deprecation warnings

src/rpc/server.cpp

@@ -18,8 +18,6 @@
 #include <boost/bind.hpp>
 #include <boost/filesystem.hpp>
 #include <boost/foreach.hpp>
-#include <boost/iostreams/concepts.hpp>
-#include <boost/iostreams/stream.hpp>
 #include <boost/shared_ptr.hpp>
 #include <boost/signals2/signal.hpp>
 #include <boost/thread.hpp>