Correct thread status callback functionality.

At the start-up, there was 2 NSAPI_STATUS_CONNECTING callbacks,
so extra one removed from ThreadInterface.cpp.
At the network lost case, there was NSAPI_STATUS_DISCONNECTED and
NSAPI_STATUS_LOCAL_UP callbacks. NSAPI_STATUS_DISCONNECTED has been removed,
since the NSAPI_STATUS_LOCAL_UP is enought.

features/nanostack/mbed-mesh-api/source/ThreadInterface.cpp

@@ -147,9 +147,6 @@ nsapi_error_t Nanostack::ThreadInterface::bringup(bool dhcp, const char *ip,
     // -end devices will get connectivity once attached to existing network
     // -devices without network settings gets connectivity once commissioned and attached to network
     _connect_status = NSAPI_STATUS_CONNECTING;
-    if (_connection_status_cb) {
-        _connection_status_cb(NSAPI_EVENT_CONNECTION_STATUS_CHANGE, NSAPI_STATUS_CONNECTING);
-    }
     if (_blocking) {
         int32_t count = connect_semaphore.wait(osWaitForever);
 

features/nanostack/mbed-mesh-api/source/thread_tasklet.c

@@ -245,10 +245,7 @@ void thread_tasklet_poll_network_status(void *param)
             }
         }
     } else {
-        if (thread_tasklet_data_ptr->connection_status != MESH_DISCONNECTED &&
-                thread_tasklet_data_ptr->connection_status != MESH_BOOTSTRAP_STARTED) {
-            thread_tasklet_network_state_changed(MESH_DISCONNECTED);
-        }
+        thread_tasklet_data_ptr->connection_status = MESH_DISCONNECTED;
     }
 }