Ingress Gateway Config not purged from Consul when job is purged

Ingress gateway config auto-generated by Nomad is not removed from Consul after job is purged:

```
~$ consul config list -kind "ingress-gateway"
consul-ingress
ig2-ingress
```

**To Reproduce**

1 - Create the following ingress gateway job:

```hcl
job "ig2" {
datacenters = ["dc1"]

group "ingress-group2" {

network {
mode = "bridge"

port "inbound" {
static = 7070
to = 7070
}
}

service {
name = "ig2-ingress"
port = "10000"

connect {
gateway {

proxy {
}

ingress {
listener {
port = 7070
protocol = "http"
service {
name = "foo"
hosts = ["some-ip","foo.ingress.dc1.consul"]
}
}
}
}
}
}
}
}
```

2 - Run the job.

3 - Run the following Consul command: `consul config list -kind "ingress-gateway":

```
~$ consul config list -kind "ingress-gateway"
ig2-ingress
```

Nomad created the config in Consul

4 - Purge the job by running `nomad stop -purge ig2`.

5 - Run `consul config list -kind "ingress-gateway"` command again:

```
~$ consul config list -kind "ingress-gateway"
ig2-ingress
```

**Expected behavior**

Configuration should have been cleaned-up since it's no longer used. Ingress gateway service is deregistered from Consul when job is purged but configuration in Consul is not deleted.

**Impact**

Over time we may end up with a lot of stale configuration in Consul

**Environment:**:

- Nomad version: 1.0.0
- Consul 1.9.0
- Envoy 1.16.0

Reproduce the lifecycle with the ingress gateway job, `nomad stop -purge ig2`, and `consul config list -kind "ingress-gateway"`. Trace Nomad's Consul integration for ingress gateway configuration cleanup; done means the generated `ig2-ingress` configuration is absent from Consul after the job is purged.

Written by the indexing model from the issue text.