fix race condition when umounting

this part of code raise a race condition, where previous container start
failed and call unmount, where after the next container starts and
unmount removes the mount entry.(gc takes 1 second after calling unmount).
We don't need to call umount in this case as the gc will always run if container
is being removed  which gc thread will clean unused mount entries.

core/compute/compute.go

@@ -29,8 +29,6 @@ func DoInstanceActivate(instance model.Instance, host model.Host, progress *prog
 		return errors.Wrap(err, constants.DoInstanceActivateError+"failed to get image tag")
 	}
 
-	started := false
-
 	instanceName := instance.Name
 	parts := strings.Split(instance.UUID, "-")
 	if len(parts) == 0 {
@@ -80,11 +78,6 @@ func DoInstanceActivate(instance model.Instance, host model.Host, progress *prog
 		return errors.Wrap(err, constants.DoInstanceActivateError+"failed to set up volumes")
 	}
 
-	defer func() {
-		if !started {
-			unmountRancherFlexVolume(instance)
-		}
-	}()
 	if err := setupRancherFlexVolume(instance, &hostConfig); err != nil {
 		return errors.Wrap(err, constants.DoInstanceActivateError+"failed to set up rancher flex volumes")
 	}
@@ -142,7 +135,6 @@ func DoInstanceActivate(instance model.Instance, host model.Host, progress *prog
 	}
 
 	logrus.Infof("rancher id [%v]: Container with docker id [%v] has been started", instance.ID, containerID)
-	started = true
 	return nil
 }