Remove the AI secondary navigation (develop)

[mlaetitia]

Related to #

Proposed changes

• Remove the AI secondary navigation from the header.
• Remove its styles from _8_overrides.templates.scss, and rebuild both style.css and editor-style.css.
• Remove the ai_secondary_nav menu location.

Why are these changes being made?

The secondary nav rendered only on pages using the AI Full Width template, as a sticky bar of links below the header. It is being retired.

register_nav_menu( 'ai', ... ) stays: header-ai.php still guards on it.

Testing instructions

This is live on production: page 135639, /ai, uses the AI Full Width template and currently renders the sticky sub-navigation. Staging may differ.

• On a page using the AI Full Width template, the row of links below the header is gone. The header, page content and footer are unchanged.
• On mobile, the "Open Source AI" label above those links is gone too.
• Appearance > Menus no longer offers "AI Secondary Navigation" as a location. The menu itself is untouched and can be deleted separately.
• Other page templates are unaffected.

After this ships, the matching rules in the Customizer's Additional CSS (.ai-secondary-navbar-wrapper, .ai-secondary-nav-menu, .ai-mobile-label, .ai-secondary-mobile-menu) are dead and can be cleared out.