Revert "fixes #1565: Allow session to flush commands on an engine error"

This reverts commit 54799c8c.

This change broke the tests/test_shutdown_stress testcase.

src/session_base.cpp

@@ -429,9 +429,6 @@ void zmq::session_base_t::engine_error (
 
     if (zap_pipe)
         zap_pipe->check_read ();
-
-    zmq_assert(socket);
-    socket->flush_commands();
 }
 
 void zmq::session_base_t::process_term (int linger_)

src/socket_base.cpp

@@ -1467,13 +1467,6 @@ void zmq::socket_base_t::check_destroy ()
     }
 }
 
-void zmq::socket_base_t::flush_commands ()
-{
-    ENTER_MUTEX();
-    process_commands (0, false);
-    EXIT_MUTEX();
-}
-
 void zmq::socket_base_t::read_activated (pipe_t *pipe_)
 {
     xread_activated (pipe_);

src/socket_base.hpp

@@ -109,8 +109,6 @@ namespace zmq
         void out_event ();
         void timer_event (int id_);
 
-        void flush_commands();
-
         //  i_pipe_events interface implementation.
         void read_activated (pipe_t *pipe_);
         void write_activated (pipe_t *pipe_);