Remove redundant warnings

[pleshakov]

The warnings reported by dataplane.BuildConfiguration() are nowredundant - they are reported as Conditions in the statuses of Gateway API resources, which includes warnings-related cases for:

• invalid listener in an HTTPRoute
• unresolved backend ref an HTTPRoute

Remove the code that generates and reports warnings.

https://github.com/nginxinc/nginx-kubernetes-gateway/blob/99320a55aa253df4920d9e08f1b166afab500483/internal/state/change_processor.go#L174

Aha! Link: https://nginx.aha.io/features/NKG-55