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

- remove subscription model
- remove billing-related code

app/controllers/api/base_controller.rb

@@ -10,8 +10,4 @@ class Api::BaseController < ApplicationController
   def authenticate_by_access_token?
     request.headers[:api_access_token].present? || request.headers[:HTTP_API_ACCESS_TOKEN].present?
   end
-
-  def check_billing_enabled
-    raise ActionController::RoutingError, 'Not Found' unless ENV['BILLING_ENABLED']
-  end
 end