remove currency model and all references

spec/models/balance_spec.rb

@@ -4,21 +4,18 @@
 #
 # Table name: balances
 #
-#  id          :bigint           not null, primary key
-#  amount      :decimal(20, 10)  default(0.0), not null
-#  created_at  :datetime         not null
-#  updated_at  :datetime         not null
-#  currency_id :bigint           not null
-#  user_id     :bigint           not null
+#  id         :bigint           not null, primary key
+#  amount     :decimal(20, 10)  default(0.0), not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#  user_id    :bigint           not null
 #
 # Indexes
 #
-#  index_balances_on_currency_id  (currency_id)
-#  index_balances_on_user_id      (user_id)
+#  index_balances_on_user_id  (user_id)
 #
 # Foreign Keys
 #
-#  fk_rails_...  (currency_id => currencies.id)
 #  fk_rails_...  (user_id => users.id)
 #
 require "rails_helper"
@@ -30,7 +27,6 @@ RSpec.describe(Balance, type: :model) do
 
   describe "associations" do
     it { is_expected.to(belong_to(:user)) }
-    it { is_expected.to(belong_to(:currency)) }
   end
 
   describe ".withdrwal!" do