MXS-2314 Remove unnecessary worker states

The removed states were not used for anything useful.

server/core/poll.cc

@@ -152,10 +152,6 @@ void dShowThreads(DCB* dcb)
             state = "Stopped";
             break;
 
-        case Worker::IDLE:
-            state = "Idle";
-            break;
-
         case Worker::POLLING:
             state = "Polling";
             break;
@@ -164,10 +160,6 @@ void dShowThreads(DCB* dcb)
             state = "Processing";
             break;
 
-        case Worker::ZPROCESSING:
-            state = "Collecting";
-            break;
-
         default:
             mxb_assert(!true);
         }

server/core/routingworker.cc

@@ -625,8 +625,6 @@ void RoutingWorker::epoll_tick()
 {
     dcb_process_idle_sessions(m_id);
 
-    m_state = ZPROCESSING;
-
     delete_zombies();
 
     check_systemd_watchdog();