feat: remove billing config. (#1761)

api/.env.example

@@ -117,7 +117,3 @@ HOSTED_ANTHROPIC_API_BASE=
 HOSTED_ANTHROPIC_API_KEY=
 HOSTED_ANTHROPIC_QUOTA_LIMIT=600000
 HOSTED_ANTHROPIC_PAID_ENABLED=false
-
-# Billing configuration
-BILLING_API_URL=http://127.0.0.1:8000/v1
-BILLING_API_SECRET_KEY=