Use feature flag instead of ExPlat for land in the editor

[mkevins]

Description

This PR replaces the Land on the editor ExPlat experiment with a development feature flag. This is needed to avoid pinging the ExPlat server now that the experiment is finished

To test:

Use the test cases of the A/B test landing in the editor PR verifying that:

• when the landInTheEditor flag is disabled the control flow applies
• when the landInTheEditor flag is enabled the treatment flow applies

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

You can test the Jetpack changes on this Pull Request by downloading it from AppCenter here with build number: pr18189-9d864f0. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[wpmobilebot]

You can test the WordPress changes on this Pull Request by downloading it from AppCenter here with build number: pr18189-9d864f0. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[mkevins]

I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary

I believe this should not be user-facing, since the experiment has ended. If this assumption is not valid, a release note should be added before merging.

[antonis]

Thank for tackling this @mkevins 🙇
I tested the app and it works as expected. The code changes also look good 🎉