Remove unnecessary code

A descriptor is always added to the global epoll instance or
to a specific worker, never just to _any_ worker.

include/maxscale/poll_core.h

@@ -60,11 +60,6 @@ typedef struct mxs_poll_data
  */
 #define MXS_WORKER_ALL -1
 
-/**
- * A file descriptor should be added to the poll set of some worker.
- */
-#define MXS_WORKER_ANY -2
-
 /**
  * Add a file descriptor with associated data to the poll set.
  *