remove currency model and all references

spec/factories/sell_crypto_orders.rb

@@ -10,23 +10,19 @@
 #  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_sell_crypto_orders_on_currency_id  (currency_id)
-#  index_sell_crypto_orders_on_user_id      (user_id)
+#  index_sell_crypto_orders_on_user_id  (user_id)
 #
 # Foreign Keys
 #
-#  fk_rails_...  (currency_id => currencies.id)
 #  fk_rails_...  (user_id => users.id)
 #
 FactoryBot.define do
   factory :sell_crypto_order do
     association :user
-    association :currency
     status { :processing }
     received_amount_cents { rand(10000) }
     paid_amount { rand * 10000 }