Chore: Remove dead code related to billing (#935)

- remove subscription model
- remove billing-related code

.env.example

@@ -89,15 +89,6 @@ TWITTER_ENVIRONMENT=
 ## Mobile app env variables
 IOS_APP_ID=6C953F3RX2.com.chatwoot.app
 
-####  This environment variables are only required in hosted version which has billing
-ENABLE_BILLING=
-
-## chargebee settings
-CHARGEBEE_API_KEY=
-CHARGEBEE_SITE=
-CHARGEBEE_WEBHOOK_USERNAME=
-CHARGEBEE_WEBHOOK_PASSWORD=
-
 ## Push Notification
 ## generate a new key value here : https://d3v.one/vapid-key-generator/
 # VAPID_PUBLIC_KEY=