Fix staticcheck warning (#726)

```console
$ staticcheck ./...
worker_test.go:21:2: package "github.com/zalando-incubator/kube-ingress-aws-controller/aws" is being imported more than once (ST1019)
        worker_test.go:22:2: other import of "github.com/zalando-incubator/kube-ingress-aws-controller/aws"
```

Signed-off-by: Alexander Yastrebov <[email protected]>