Remove old polling message system

The old polling message system is obsolete now that the worker messages
are implemented. The old system was only used to clean up the persistent
connection pool of a server.

server/core/maxscale/poll.h

@@ -61,6 +61,4 @@ void            dShowEventStats(DCB *dcb);
 int             poll_get_stat(POLL_STAT stat);
 RESULTSET       *eventTimesGetList();
 
-void            poll_send_message(enum poll_message msg, void *data);
-
 MXS_END_DECLS