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

- remove subscription model
- remove billing-related code

app/javascript/dashboard/assets/scss/_woot.scss

@@ -3,7 +3,6 @@
 @import 'animations';
 
 @import 'foundation-custom';
-@import 'widgets/billing';
 @import 'widgets/buttons';
 @import 'widgets/conv-header';
 @import 'widgets/conversation-card';