remove extra_startup_steps from app config

This should have been done as part of bug17880.

Author: Matthias Radestock

ebin/rabbit_app.in

@@ -17,7 +17,6 @@
   {env, [{tcp_listeners, [{"0.0.0.0", 5672}]},
          {ssl_listeners, []},
          {ssl_options, []},
-         {extra_startup_steps, []},
          {default_user, <<"guest">>},
          {default_pass, <<"guest">>},
          {default_vhost, <<"/">>},