Honor runners.autoscaler.delete_instances_on_shutdown

Gitlab Runner itself has a config parameter (runners.autoscaler.delete_instances_on_shutdown)
which can be used to remove instances on runner shutdown. There is no
need to handle this ourself, Gitlab Runner (or better taskscaler) will
mark all instances to be removed before shutting down.

See
* https://gitlab.com/gitlab-org/fleeting/taskscaler/-/blob/f3cdb5de12a49180a8da538decd0e023b62e3a41/options.go#L220-L228
* https://gitlab.com/gitlab-org/fleeting/taskscaler/-/blob/f3cdb5de12a49180a8da538decd0e023b62e3a41/taskscaler.go#L273

Fixes #43

provider.go

@@ -390,25 +390,5 @@ func (g *InstanceGroup) ConnectInfo(ctx context.Context, instanceID string) (pro
 }
 
 func (g *InstanceGroup) Shutdown(ctx context.Context) error {
-
-	// Not necessary, after reboot all old nodes will be deleted anyway.
-	// But that speedup that process.
-	instances, err := g.getInstances(ctx)
-	if err != nil {
-		return err
-	}
-
-	for _, srv := range instances {
-		lg := g.log.With("id", srv.ID, "name", srv.Name)
-
-		err2 := g.deleteInstance(ctx, srv.ID)
-		if err2 != nil {
-			lg.Error("Failed to delete instance", "err", err2)
-			err = errors.Join(err, err2)
-		}
-
-		lg.Info("Deleted instance on shutdown")
-	}
-
-	return err
+	return nil
 }