fix stray UNIMPL from (now implemented) DISPOSE handler

maestro/pool_manager.c

@@ -1723,7 +1723,7 @@ mstro_pm__handle_dispose_phase3(mstro_event event,
   DEBUG("DISPOSE phase 3/3, event %p\n", event);
   mstro_pm__msg_free(cont->msg);
   mstro_stats_add_counter(MSTRO_STATS_CAT_PROTOCOL, MSTRO_STATS_L_PM_NUM_DISPOSE, 1);
-  return MSTRO_UNIMPL;
+  return MSTRO_OK;
 }