Remove our custom defaulter when controller-runtime fix is released

[mimowo]

What would you like to be cleaned:

We have a custom defaulter in Kueue registered via pkg/controller/jobframework/webhook/builder.go to workaround a bug in controller-runtime which was dropping unknown fields in webhooks.

For that we need to await until the fix kubernetes-sigs/controller-runtime#2982 is released in controller-runtime, probably 0.20.

Why is this needed:

To cleanup Kueue code, and use a fix which allows to also remove fields in Kueue's webhooks (if we need it).

[mimowo]

cc @tenzen-y @mbobrovskyi