refactor(admin): remove download config settings page and chain

- Drop the download config menu entry, route and schema from the settings SPA
- Remove the PHP schema section, the server-side alias validation and the
  reserved-alias list that only served that section
- Unregister aksrt_download_config from standalone import/export keys
- Drop the contract tests that locked the removed page; the Download.php
  runtime contracts stay untouched

assets/admin/src/constants/sections/permissions.js

@@ -47,15 +47,5 @@ export const permissionsSection = {
       canonicalDomain: 'permissions',
       section: 'attachment',
     }),
-    schemaPage({
-      key: 'permissions-download',
-      name: 'PermissionsDownload',
-      label: '下载配置',
-      route: '/permissions/download',
-      title: '下载配置',
-      desc: '下载链接别名、默认使用次数与有效期限规则。',
-      canonicalDomain: 'permissions',
-      section: 'download',
-    }),
   ],
 }