[feat: gw api] support gw deletion

[zac-nixon]

Description

Originally, we missed adding the deletion logic to Gateways, meaning that the AWS resources would not get cleaned up after Gateway deletion.

• Added deletion logic
• Refactored gateway controller to use gateway specific events
• Removed redundant LB config resolver function

Testing:

• Deleted an NLB and ALB gateway.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[shraddhabang]

/lgtm
/approved

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shraddhabang, zac-nixon

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