remove currency model and all references

spec/models/buy_crypto_order_spec.rb

@@ -10,17 +10,14 @@
 #  status            :string           not null
 #  created_at        :datetime         not null
 #  updated_at        :datetime         not null
-#  currency_id       :bigint           not null
 #  user_id           :bigint           not null
 #
 # Indexes
 #
-#  index_buy_crypto_orders_on_currency_id  (currency_id)
-#  index_buy_crypto_orders_on_user_id      (user_id)
+#  index_buy_crypto_orders_on_user_id  (user_id)
 #
 # Foreign Keys
 #
-#  fk_rails_...  (currency_id => currencies.id)
 #  fk_rails_...  (user_id => users.id)
 #
 require "rails_helper"
@@ -51,6 +48,5 @@ RSpec.describe(BuyCryptoOrder, type: :model) do
 
   describe "associations" do
     it { is_expected.to(belong_to(:user)) }
-    it { is_expected.to(belong_to(:currency)) }
   end
 end