Review Mozilla 2012344: Hide main AI settings when the feature is blocked

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=2012344

Description

See !1913 (comment 3370266). Made some changes to preferences.

In particular, I wonder if we can short-circuit canShowAiFeature to simply return false, as a way to hide a few upstream settings that are outside the "ai" pane. Or just use browser.ai.control.default (#44541).

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