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.

functions.php

@@ -45,11 +45,8 @@ if (!function_exists('aksrt_get_setting')) {
             return AKSRT_Settings::get_all();
         }
 
-        $legacy = get_option('aksrt_settings', array());
-        $seo    = get_option('aksrt_seo_settings', array());
-        $basic  = get_option('aksrt_basic_settings', array());
-
-        return array_merge($legacy, $seo, $basic);
+        $settings = get_option('aksrt_settings', array());
+        return is_array($settings) ? $settings : array();
     }
 
     /**