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

- remove subscription model
- remove billing-related code

app/controllers/api_controller.rb

@@ -1,6 +1,5 @@
 class ApiController < ApplicationController
   skip_before_action :set_current_user, only: [:index]
-  skip_before_action :check_subscription, only: [:index]
 
   def index
     render json: { version: Chatwoot.config[:version], timestamp: Time.now.utc.to_formatted_s(:db) }