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
pd_subnets dict.
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.
Change-Id: I6e6d890c196716c0ef4bcc2922f1ec4c142a6e79
Closes-Bug: #1892364