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

- remove subscription model
- remove billing-related code

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_06_06_132552) do
+ActiveRecord::Schema.define(version: 2020_06_07_140737) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
@@ -328,18 +328,6 @@ ActiveRecord::Schema.define(version: 2020_06_06_132552) do
     t.index ["user_id"], name: "index_notifications_on_user_id"
   end
 
-  create_table "subscriptions", id: :serial, force: :cascade 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
-
   create_table "super_admins", force: :cascade do |t|
     t.string "email", default: "", null: false
     t.string "encrypted_password", default: "", null: false