cluster/controllers/plugin: remove unused Controller.taskID (unused)

```
daemon/cluster/controllers/plugin/controller.go:37:2: U1000: field `taskID` is unused (unused)
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/cluster/controllers/plugin/controller.go

@@ -34,7 +34,6 @@ type Controller struct {
 
 	pluginID  string
 	serviceID string
-	taskID    string
 
 	// hook used to signal tests that `Wait()` is actually ready and waiting
 	signalWaitReady func()