interface-id should be empty in subnet and not copied from shared-network if not specified directly

shared network is defined this way:

{'arguments': {'remote': {'type': 'mysql'},
               'server-tags': ['default'],
               'shared-networks': [{'interface-id': 'vlan-a',
                                    'name': 'floor13'}]},

subnet this way:

{'arguments': {'remote': {'type': 'mysql'},
               'server-tags': ['default'],
               'subnets': [{'id': 1,
                            'interface': '',
                            'pools': [{'pool': '2001:db8:1::1-2001:db8:1::100'}],
                            'shared-network-name': 'floor13',
                            'subnet': '2001:db8:1::/64'}]},

then config-get returns this:

...
"shared-networks": [
          {
            "interface-id": "vlan-a",
            "name": "floor13",
            "option-data": [],
            "relay": {
              "ip-addresses": []
            },
            "subnet6": [
              {
                "id": 1,
                "interface-id": "vlan-a",
                "option-data": [],
                "pd-pools": [],
                "pools": [
                  {
                    "option-data": [],
                    "pool": "2001:db8:1::1-2001:db8:1::100"
                  }
                ],
                "relay": {
                  "ip-addresses": []
                },
                "reservations": [],
                "subnet": "2001:db8:1::/64"
              }
            ]
          }
        ],
...

and the problem is that interface-id in subnet is "vlan-a" while it was not set in subnet-set command. This field should not be present in subnet at all.

Test in Forge: tests/dhcpv4/kea_only/config_backend/test_interface_id.py::test_interface_id_in_network