Remove MULE_SESSION from outbound JMS messages.

The MULE_SESSION message in outbound JMS messages is not compatible
between Mule versions 3.2 and 3.4 exchanging messages across the same
JMS server.  Remove it.

See
http://forum.mulesoft.org/mulesoft/topics/_mule_user_malformed_session_variable_error
http://doctorjw.wordpress.com/2012/08/27/jms-messages-between-mule-versions/