In the RouterInfo._process_internal_ports() method when it process
old ports and port belongs to the subnet with CIDR assigned by
Prefix Delegation it will try to remove subnet_id key from the
However it seems that in some case it may happen that such subnet_id key
is not added to the pd_subnets dict and processing of ports fails.
We shouldn't fail in such case, if there is no subnet_id key in this
dict we should be good as we want to delete it simply. So this patch
changes that to not raise KeyError in such case.
(cherry picked from commit