When subnet is moved into a shared network the old subnet entry is not removed

I did the following test:

• Started Kea with a single subnet and without any shared networks
• I let Stork pull the subnet and store in its database
• Stopped Kea and moved the subnet into a new shared network
• Started Kea again and let Stork pull the updated configuration
• Navigated to the list of subnets
• I now see two instances of the same subnet, one under shared network and one global.