chore: rm custom CODEOWNERS logic

[colbywhite]

This removes the custom CODEOWNERS logic that results in assignees being added to PRs. The standard GitHub logic around CODEOWNERS already does this, but with required reviewers instead.

From what I can tell, there is no advantage of assignee over required reviewer.

• The required reviewer logic gates the PRs, which is the most tangible thing we want out of CODEOWNERS. Assignees does not gate.
• When filtering the big list of PRs in the repo, you can can filter by either assignee, reviewer (i.e. "Awaiting review from you or your team"), or both. There may be historical context where it either wasn't obvious that you could filter on reviewers or that didn't exist in the GitHub UI and thus a reliance on assignee was formed. Also, GitHub's the root PR page does the same filtering (with arguably better UI).
• There are several places in the GitHub UI where a small shield is displayed. Hovering/clicking these shields typically give you information related to the CODEOWNERS context, i.e. why a person was added as a reviewer, who owns the file you're looking at, etc.) This shield isn't very obvious in the UI admittedly, but it is there. There's also explicit "X requested review from Y" language that gets added by GitHub in an another attempt to make the context obvious, although the wording isn't the greatest.

If the main benefit of this custom logic is that the assignee field gets marginally better treatment from the GitHub UI than reviewer, it makes sense to nuke this and rely on GitHub's standard CODEOWNER functionality. This avoids the ongoing cost of maintenance for this code for a small benefit. I'll be giving a quick walkthrough of the oddities in GitHub's UI to the team to ease adoption.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/.github/CODEOWNERS	@cloudflare/pcx-technical-writing, @cloudflare/content-engineering, @kodster28
/.github/	@cloudflare/pcx-content-engineering, @cloudflare/content-engineering, @kodster28
*.js	@cloudflare/pcx-content-engineering, @cloudflare/content-engineering, @kodster28
*.ts	@cloudflare/pcx-content-engineering, @cloudflare/content-engineering, @kodster28

[github-actions]

Preview URL: https://6479fed7.preview.developers.cloudflare.com
Preview Branch URL: https://codeowner-test.preview.developers.cloudflare.com