Remove of ZMQ_IDENTITY_FD socket option

Revert "linking fd to pipe identity via socket option"

This reverts commit fe3e8c5c70.

Conflicts:
	include/zmq.h
	src/pipe.hpp
	src/session_base.cpp

src/stream_engine.hpp

@@ -76,8 +76,6 @@ namespace zmq
         void out_event ();
         void timer_event (int id_);
 
-        // export s via i_engine so it is possible to link a pipe to fd
-        fd_t get_assoc_fd (){ return s; };
     private:
 
         //  Unplug the engine from the session.