Fix gometalint errors

Signed-off-by: Manish Tomar <manish.tomar@docker.com>

notifications/sinks.go

@@ -284,11 +284,6 @@ type retryingSink struct {
 	}
 }
 
-type retryingSinkListener interface {
-	active(events ...Event)
-	retry(events ...Event)
-}
-
 // TODO(stevvooe): We are using circuit break here, which actually doesn't
 // make a whole lot of sense for this use case, since we always retry. Move
 // this to use bounded exponential backoff.