Hide (again) the "Allow in private browsing" checkboxes from WebExtension management UI when in global PBM

Following up #43915 (comment 3246331) - which is for Android, but the problem applies to desktop as well.

Since in global PBM world no extension can properly work if "Allow in private browsing" is not enabled, we should hide the footgun UI to disable it (as we did in the past).

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