api: make staticchecker happy

```
Running: golangci-lint run --new-from-patch=2386.patch
api/model.go:2061:23: SA1019: im.Status.Instances is deprecated: Replaced by InstanceEngines and InstanceReplicas +optional +nullable (staticcheck)
		Instances:          im.Status.Instances,
		                    ^
```

Longhorn 7095

Signed-off-by: Derek Su <derek.su@suse.com>

api/model.go

@@ -501,7 +501,6 @@ type InstanceManager struct {
 	InstanceEngines  map[string]longhorn.InstanceProcess `json:"instanceEngines"`
 	InstanceReplicas map[string]longhorn.InstanceProcess `json:"instanceReplicas"`
 
-	// Deprecated
 	Instances map[string]longhorn.InstanceProcess `json:"instances"`
 }