XD-1222 Remove duplicate MBeanServer def

https://jira.springsource.org/browse/XD-1222
Since the 'jmxEnabled' flag is shared by admin/launcher and
the module's MBeanExportingPlugin, the definition of MBeanServer
by the module's plugin becomes duplicate as the MBeanServer
is already defined as XD global beans (xd-global-beans.xml)