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/socket_base.hpp

@@ -133,13 +133,10 @@ namespace zmq
 
         //  The default implementation assumes there are no specific socket
         //  options for the particular socket type. If not so, override this
-        //  methods.
+        //  method.
         virtual int xsetsockopt (int option_, const void *optval_,
             size_t optvallen_);
 
-        virtual int xgetsockopt (int option_, const void *optval_,
-            size_t *optvallen_);
-
         //  The default implementation assumes that send is not supported.
         virtual bool xhas_out ();
         virtual int xsend (zmq::msg_t *msg_);