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/session_base.cpp

@@ -367,9 +367,7 @@ void zmq::session_base_t::process_attach (i_engine *engine_)
         //  Remember the local end of the pipe.
         zmq_assert (!pipe);
         pipe = pipes [0];
-        // Store engine assoc_fd for linking pipe to fd
-        pipe->assoc_fd = engine_->get_assoc_fd ();
-        pipes [1]->assoc_fd = pipe->assoc_fd;
+
         //  Ask socket to plug into the remote end of the pipe.
         send_bind (socket, pipes [1]);
     }