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