Content Types: Remove "show in REST API"

[im3dabasia]

What?

Related: #77600

Removes the Show in REST API toggle from the taxonomy visibility controls.

Why?

Allowing users to disable show_in_rest on a user-defined taxonomy is dangerous, it silently breaks REST API access and block editor support for that taxonomy.

How?

1. Removed the Show in REST API field from the taxonomy visibility form.
2. Removed the E2E test that verified toggling it off blocked REST access (the control no longer exists).

Testing Instructions

1. Enable the gutenberg-content-types experiment.
2. Create a user taxonomy and open the Visibility panel.
3. Confirm there is no Show in REST API toggle.
4. Run npm run test:e2e -- test/e2e/specs/admin/user-taxonomies.spec.js and confirm all remaining tests pass.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: im3dabasia <im3dabasia1@git.wordpress.org>
Co-authored-by: desrosj <desrosj@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>
Co-authored-by: cbravobernal <cbravobernal@git.wordpress.org>
Co-authored-by: retlehs <retlehs@git.wordpress.org>
Co-authored-by: simison <simison@git.wordpress.org>
Co-authored-by: jonathan-dejong <jonathandejong@git.wordpress.org>
Co-authored-by: jhmonroe <jhmonroe@git.wordpress.org>
Co-authored-by: ts-mz <tschwarz-1@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.