remove currency model and all references

app/graphql/inputs/create_buy_crypto_order_attributes_input.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 module Inputs
   class CreateBuyCryptoOrderAttributesInput < Types::BaseInputObject
-    argument :currency_id, ID, required: true
     argument :amount_cents, Integer, "Amount to be paid", required: true
   end
 end

app/graphql/inputs/create_sell_crypto_order_attributes_input.rb

@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 module Inputs
   class CreateSellCryptoOrderAttributesInput < Types::BaseInputObject
-    argument :currency_id, ID, required: true
     argument :amount, String, "Amount to be paid", required: true
   end
 end