[manila] Share network subnets unclean

Hi Manila core team We have an issue with share network teardown that leaves subnet unclean when no router information is provided. This led to a huge list of subnets stay unclean with manila CSI plugin in Kubernetes. The fix is here: https://review.opendev.org/c/openstack/manila/+/955996 It would be appreciated if you could kindly provide your review on this. Thank you *Rico Lin*