Remove Enable AI header toggle

[wprashed]

Fixes #600

Description

The Enable AI header toggle displayed a binary ON/OFF state that did not accurately reflect individual feature configurations. Per maintainer consensus in #600, this PR removes the global header toggle entirely, allowing individual feature and experiment group toggles (and deactivating the plugin) to manage functionality.

Changes Included

• Removed the Enable AI header <ToggleControl> and InfoTip from routes/ai-home/stage.tsx.
• Removed unused globalToggleDescription variable in stage.tsx.
• Updated E2E test specs (settings.spec.js, import-export.spec.js, helpers.ts) to remove references to the Enable AI toggle.

Unlinked contributors: manueldiaz2006t-blip.

Co-authored-by: dhrupo dhrupo@git.wordpress.org
Co-authored-by: dkotter dkotter@git.wordpress.org
Co-authored-by: jeffpaul jeffpaul@git.wordpress.org
Co-authored-by: gziolo gziolo@git.wordpress.org

Testing Instructions

1. Go to the AI settings page (options-general.php?page=ai-wp-admin).
2. Confirm the header renders cleanly with Docs, Contribute, and options links, without the Enable AI header toggle.
3. Confirm section group toggles ("Enable all" / "Disable all") and individual feature toggles continue to work as expected.

[github-actions]

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Unlinked Accounts

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Unlinked contributors: manueldiaz2006t-blip.

Co-authored-by: wprashed <wprashed@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: andreilupu <euthelup@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.57%. Comparing base (0833eec) to head (62dcac0).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #985   +/-   ##
==========================================
  Coverage      74.57%   74.57%           
  Complexity      3132     3132           
==========================================
  Files            132      132           
  Lines          12213    12213           
==========================================
  Hits            9108     9108           
  Misses          3105     3105           

Flag	Coverage Δ
unit	74.57% <ø> (ø)

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[dkotter]

E2E tests are failing here.

In addition, please ensure the PR description contains everything from our PR template, as already mentioned here