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

- remove subscription model
- remove billing-related code

db/migrate/20200607140737_remove_subscriptions.rb

@@ -0,0 +1,15 @@
+class RemoveSubscriptions < ActiveRecord::Migration[6.0]
+  def change
+    drop_table :subscriptions do |t|
+      t.string 'pricing_version'
+      t.integer 'account_id'
+      t.datetime 'expiry'
+      t.string 'billing_plan', default: 'trial'
+      t.string 'stripe_customer_id'
+      t.datetime 'created_at', null: false
+      t.datetime 'updated_at', null: false
+      t.integer 'state', default: 0
+      t.boolean 'payment_source_added', default: false
+    end
+  end
+end