HTTPRoute status updater does not remove non-targeted Gateway

*Description*:
>What issue is being seen? Describe what should be happening instead of
the bug, for example: The expected value isn't returned, etc.

If the HTTPRoute targeted a Gateway resource (`spec.parentRefs`) but then it does not target it anymore, the `status.parents` still has reference to the non-targeted Gateway in it.

In the example below, the `gateway-notfound` is not in the `spec.parentRefs` anymore, but is still present in the `status.parents`.

```
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
...
spec:
parentRefs:
- group: gateway.networking.k8s.io
kind: Gateway
name: gateway-1
namespace: envoy-gateway-ns
sectionName: https
...
status:
parents:
- conditions:
...
parentRef:
group: gateway.networking.k8s.io
kind: Gateway
name: gateway-notfound
namespace: envoy-gateway-ns
sectionName: https
- conditions:
...
parentRef:
group: gateway.networking.k8s.io
kind: Gateway
name: gateway-1
namespace: envoy-gateway-ns
sectionName: https
```

Found it through the EG dashboards, where Targeted Parents show 8 but the Attached Parents is a higher number (9)

*Repro steps*:
> Include sample requests, environment, etc. All data and inputs
required to reproduce the bug.

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*Environment*:
>Include the environment like gateway version, envoy version and so on.

*Logs*:
>Include the access logs and the Envoy logs.