chore(admin-web/login): drop redundant tenant admin login subheading

The 欢迎回来 headline already conveys context; the secondary
"Tenant Admin Login" line added noise on the simplified login card.
Remove the paragraph and the now-unused i18n key.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/i18n/messages/en-US.ts

@@ -91,7 +91,6 @@ const enUS = {
   },
   auth: {
     welcomeBack: "Welcome back",
-    tenantAdminLogin: "Tenant Admin Login",
     loginAndEnter: "Sign in to workspace",
     email: "Email",
     loginIdentifier: "Phone / Email",