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

- remove subscription model
- remove billing-related code

spec/controllers/api/v1/accounts/subscriptions_controller_spec.rb

@@ -1,46 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe 'Subscriptions API', type: :request do
-  let(:account) { create(:account) }
-
-  describe 'GET /api/v1/accounts/{account.id}/subscriptions' do
-    context 'when it is an unauthenticated user' do
-      it 'returns unauthorized' do
-        ENV['BILLING_ENABLED'] = 'true'
-
-        get "/api/v1/accounts/#{account.id}/subscriptions"
-
-        expect(response).to have_http_status(:unauthorized)
-
-        ENV['BILLING_ENABLED'] = nil
-      end
-    end
-
-    context 'when it is an authenticated user' do
-      let(:agent) { create(:user, account: account, role: :agent) }
-
-      it 'returns all subscriptions' do
-        ENV['BILLING_ENABLED'] = 'true'
-
-        get "/api/v1/accounts/#{account.id}/subscriptions",
-            headers: agent.create_new_auth_token,
-            as: :json
-
-        expect(response).to have_http_status(:success)
-        expect(JSON.parse(response.body)).to eq(account.subscription_data.as_json)
-
-        ENV['BILLING_ENABLED'] = nil
-      end
-
-      it 'throws 404 error if env variable is not set' do
-        ENV['BILLING_ENABLED'] = nil
-
-        get "/api/v1/accounts/#{account.id}/subscriptions",
-            headers: agent.create_new_auth_token,
-            as: :json
-
-        expect(response).to have_http_status(:not_found)
-      end
-    end
-  end
-end