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

This reverts commit 54799c8c6f.

This change broke the tests/test_shutdown_stress testcase.

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_);