Do not remove databuffer on close

If exiting from a shell session, the databuffer will contain needed info for the client
to work properly.  Preserve databuffer existence.  Responsibility for deleting the
object should be in the hands of the caller.

confluent_server/confluent/consoleserver.py

@@ -535,7 +535,6 @@ class ConsoleSession(object):
     def destroy(self):
         if self.registered:
             self.conshdl.detachsession(self)
-        self.databuffer = None
         if self._evt:
             self._evt.send()
         self._evt = None