refactor(admin): retire settings migration and compat menu demotion

Stop auto-running AKSRT_Settings_Migration, drop multi-option settings
merge and Zibll favicon aliases, remove the migration tools entry from
menus/schema, flatten admin sections so pages are not demoted into a
compat bag, and retire the migration-status payload for leftover SPA
routes after a full admin rebuild.

inc/classes/class-aksrt-settings.php

@@ -222,28 +222,7 @@ class AKSRT_Settings {
     /** 获取Favicon URL */
     public static function custom_favicon_url(): string {
         $favicon = (string) self::get_value('site_favicon', '');
-        if ($favicon !== '') {
-            return $favicon;
-        }
-
-        // zibll 别名回退
-        $zibll = trim((string) get_option('favicon', ''));
-        if ($zibll !== '') {
-            return $zibll;
-        }
-
-        // zibll iconpng 别名回退
-        $iconpng = trim((string) get_option('iconpng', ''));
-        if ($iconpng !== '') {
-            return $iconpng;
-        }
-
-        $theme_mod = trim((string) get_theme_mod('aksrt_favicon', ''));
-        if ($theme_mod !== '') {
-            return $theme_mod;
-        }
-
-        return '';
+        return $favicon !== '' ? $favicon : '';
     }
 
     public static function favicon_url(): string {