removed a duplicate and incorrect unlock

Signed-off-by: Madhu Venugopal <madhu@docker.com>

libnetwork/sandboxdata.go

@@ -228,7 +228,6 @@ func (c *controller) LeaveAll(id string) error {
 	c.Unlock()
 
 	if !ok {
-		c.Unlock()
 		return fmt.Errorf("could not find sandbox for container id %s", id)
 	}