remove currency model and all references

app/models/balance.rb

@@ -4,28 +4,24 @@
 #
 # 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)
 #
 class Balance < ApplicationRecord
   include Trackable
 
   belongs_to :user
-  belongs_to :currency
 
   validates :amount, presence: true, numericality: { greater_than_or_equal_to: 0 }