Respects that NGINX's defaultbackend component is disabled by default,
rather than intentionally enabling it.

Further context:

It looks like that component is actually disabled by default upstream.
In gitlab-org/charts/gitlab!1690 (merged), there was
gitlab-org/charts/gitlab!1690 (52b9e8c0), which explicitly enabled
defaultbackend because it was inherently enabled prior to that change
(although I can't find anything yet to confirm that). On that note, the
defaultbackend image doens't seem to be NGINX-specific based on its path
in the values file:

```yaml
registry: registry.k8s.io
image: defaultbackend-amd64
```

Changelog: changed