fix(controller): remove duplicate addOrUpdateSubnetQueue.Add in handleAddOrUpdateVpc

Cherry-pick of kubeovn#6268 introduced a duplicate unconditional call to
addOrUpdateSubnetQueue.Add alongside the IsNotReady() conditional call,
causing every subnet to be enqueued twice on VPC update.

Restore correct behavior matching release-1.15: only enqueue when
subnet is not ready.

Author: zbb88888

pkg/controller/vpc.go

@@ -607,7 +607,6 @@ func (c *Controller) handleAddOrUpdateVpc(key string) error {
 			if subnet.Status.IsNotReady() {
 				c.addOrUpdateSubnetQueue.Add(subnet.Name)
 			}
-			c.addOrUpdateSubnetQueue.Add(subnet.Name)
 			if vpc.Name != util.DefaultVpc && vpc.Spec.EnableBfd && subnet.Spec.EnableEcmp {
 				custVpcEnableExternalEcmp = true
 			}