socket_base: process_unplug () is not used, remove it

src/socket_base.cpp

@@ -837,10 +837,6 @@ void zmq::socket_base_t::process_bind (pipe_t *pipe_)
     attach_pipe (pipe_);
 }
 
-void zmq::socket_base_t::process_unplug ()
-{
-}
-
 void zmq::socket_base_t::process_term (int linger_)
 {
     //  Unregister all inproc endpoints associated with this socket.

src/socket_base.hpp

@@ -180,7 +180,6 @@ namespace zmq
         //  Handlers for incoming commands.
         void process_stop ();
         void process_bind (zmq::pipe_t *pipe_);
-        void process_unplug ();
         void process_term (int linger_);
 
         //  Socket's mailbox object.