ExPlat: Remove A/B experiment

[renanferrari]

This PR removes the A/B experiment added in #14286, as it has already been completed.

The code changes should be pretty straightforward.

To test

Make sure all existing unit tests pass. Additionally:

1. Open the app and make sure you've selected a site that supports Stories.
2. On the My Site screen, tap the FAB.
3. Notice that the menu actions have the following order: "Blog post", "Site page" and "Story post".

Regression Notes

1. Potential unintended areas of impact
   Main creation bottom sheet.

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

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• 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.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[planarvoid]

Looks good and works as expected 👍