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

- remove subscription model
- remove billing-related code

app/views/layouts/vueapp.html.erb

@@ -31,7 +31,6 @@
       window.chatwootConfig = {
         hostURL: '<%= ENV.fetch('FRONTEND_URL', '') %>',
         fbAppId: '<%= ENV.fetch('FB_APP_ID', nil) %>',
-        billingEnabled: <%= ActiveModel::Type::Boolean.new.cast(ENV.fetch('BILLING_ENABLED', false)) %>,
         signupEnabled: '<%= ENV.fetch('ENABLE_ACCOUNT_SIGNUP', true) %>',
         <% if ENV['VAPID_PUBLIC_KEY'] %>
         vapidPublicKey: new Uint8Array(<%= Base64.urlsafe_decode64(ENV['VAPID_PUBLIC_KEY']).bytes %>),