remove currency model and all references

spec/factories/balances.rb

@@ -4,27 +4,23 @@
 #
 # 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)
 #
 FactoryBot.define do
   factory :balance do
     association :user
-    association :currency
     amount { (rand * (10000 - 0) + 0) }
   end
 end