chore(admission) remove GWAPI types from webhook

[rainest]

What this PR does / why we need it:

Remove GWAPI types from the webhook. We plan to remove these in the controller (https://github.com/Kong/kubernetes-ingress-controller/issues/5197) and there's no clear need to continue using them. Since they currently cause a version compatibility issue (https://github.com/Kong/charts/pull/954) I'd like to just remove them preemptively. KIC hasn't changed yet, but those changes aren't a hard prereq to removing the webhook config.

Held pending confirmation that we don't need these.

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] PR is based off the current tip of the main branch.
• [x] Changes are documented under the "Unreleased" header in CHANGELOG.md
• [x] New or modified sections of values.yaml are documented in the README.md
• [x] Commits follow the Kong commit message guidelines

[programmer04]

For HTTPRoute regexes are validated with webhook see

• https://github.com/Kong/kubernetes-ingress-controller/pull/4608

[rainest]

Forgot to close this after merging #954.