Removes stale code from /hosting-config for starting the Creator trial

[p-jackson]

Related to https://github.com/Automattic/dotcom-forge/issues/6638

Proposed Changes

This is a sub part of https://github.com/Automattic/dotcom-forge/issues/6638, a task to remove code related to the old endpoint for starting trials. Doing multiple PRs otherwise the diff is quite large.

There used to be a trial button in /hosting-config which was originally disabled in #88486. But it left some of the old code in place. We need to remove it so we can eventually remove the useAddHostingTrialMutation hook and API endpoint.

~The hosting config page also uses the WithOnclickTrialRequest HoC, which from the name appears to have something to do with a trial, but when I looked at the code it's unrelated to the hosting trial.~ I've removed WithOnclickTrialRequest.

Testing Instructions

• Review the code and test the hosting config page.
• There should be no changes to the hosting config page.
• Use /hosting-config to transfer a simple site to atomic.

Pre-merge Checklist

• [ ] Has the general commit checklist been followed? (PCYsg-hS-p2)
• [ ] https://wpcalypso.wordpress.com/devdocs/docs/testing/index.md for your changes?
• [ ] Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• [ ] Have you checked for TypeScript, React or other console errors?
• [ ] Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• [ ] Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• [ ] For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?