Static route key retained despite deleting all paths

[taspelund]

Building from main, I see this when manually testing adding/removing static routes:

➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static add-v4-route 1.1.1.1/32 100.64.0.0
➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static add-v4-route 1.1.1.1/32 100.64.0.1
➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static get-v4-routes
{
    "1.1.1.1/32": [
        Path {
            bgp: None,
            local_pref: None,
            nexthop: 100.64.0.0,
            shutdown: false,
            vlan_id: None,
        },
        Path {
            bgp: None,
            local_pref: None,
            nexthop: 100.64.0.1,
            shutdown: false,
            vlan_id: None,
        },
    ],
}
➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static remove-v4-routes 1.1.1.1/32 100.64.0.0
➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static get-v4-routes
{
    "1.1.1.1/32": [
        Path {
            bgp: None,
            local_pref: None,
            nexthop: 100.64.0.1,
            shutdown: false,
            vlan_id: None,
        },
    ],
}
➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static remove-v4-routes 1.1.1.1/32 100.64.0.1
➜  maghemite git:(f5dbb6a) ./target/debug/mgadm static get-v4-routes
{
    "1.1.1.1/32": [],
}

I don't think we should be retaining 1.1.1.1/32 after deletion of the last path.