discovery: simplify client initialization

retrieval/discovery/marathon.go

@@ -25,7 +25,6 @@ func NewMarathon(conf *config.MarathonSDConfig) *marathon.Discovery {
 	return &marathon.Discovery{
 		Servers:         conf.Servers,
 		RefreshInterval: time.Duration(conf.RefreshInterval),
-		Done:            make(chan struct{}),
 		Client:          marathon.FetchApps,
 	}
 }

retrieval/discovery/marathon/marathon_test.go

@@ -30,8 +30,8 @@ func newTestDiscovery(client AppListClient) (chan []*config.TargetGroup, *Discov
 	ch := make(chan []*config.TargetGroup)
 	md := &Discovery{
 		Servers: []string{"http://localhost:8080"},
+		Client:  client,
 	}
-	md.Client = client
 	return ch, md
 }