Remove va_home_preview_modal feature flag after launch

[jilladams]

Description

When homepage is hard launched, this flipper can be deprecated.

User story

Describe the audience/user, enhancement or fix, and value / outcome desired.

AS A(N) engineer I WANT to reduce technical debt and the number of feature flags in play SO THAT the build system stays optimized.

(This story is speaking for Plat where they care about the fact of Flippers, but I can't speak technically to why.)

Order of operations

• Remove from vets-website first (to NOT show widget regardless of flipper, and remove flipper references) - via #13955
• Remove from vets-api after

Acceptance criteria

• [x] I expect that the va_home_preview_modal is no longer present in vets-api code or flipper UI [edit: the flipper will remain in the flipper UI since those no longer get purged automatically. This is a Platform thing that they decided to do to avoid the bug where some flippers were being accidentally deleted.]
• [x] I expect that the homepage modal still does not appear in prod

Team

Please check the team(s) that will do this work.

• [ ] CMS Team
• [X] Public Websites
• [ ] Facilities
• [ ] User support

[jilladams]

2, unless we change scope unless we change scope to handle more tech debt.

[randimays]

Marking as complete; changes have been deployed to production and modal does not show up at / or /new-home-page. As stated above, flipper still appears in the Flipper Admin because the job to clean up unused flippers is temporarily turned off due to bugs.