chore: Remove empty /manage/ admin overview (#102988)

The admin overview page is empty now. Instead of the default view of
`/manage/` being an empty view we can show the environment page instead.

Author: markstory

static/app/router/routes.tsx

@@ -2661,7 +2661,7 @@ function buildRoutes(): RouteObject[] {
   const adminManageChildren: SentryRouteObject[] = [
     {
       index: true,
-      component: make(() => import('sentry/views/admin/adminOverview')),
+      component: make(() => import('sentry/views/admin/adminEnvironment')),
     },
     {
       path: 'relays/',

static/app/views/admin/adminLayout.tsx

@@ -18,7 +18,6 @@ export function AdminNavigation() {
           id: 'admin-system-status',
           name: 'System Status',
           items: [
-            {path: '/manage/', index: true, title: 'Overview'},
             {path: '/manage/status/environment/', title: 'Environment'},
             {path: '/manage/status/packages/', title: 'Packages'},
             {path: '/manage/status/mail/', title: 'Mail'},