Dear Maintainer,

With all accounts configured as euro, there is a situation where the
user is forced to enter an EUR/USD exchange rate.  These are the steps
to reproduce:

1) make sure EUR is the default currency for the books, and add these
   EUR-denominated accounts:

   Assets:current
   Liabilities:tax loan
   Expenses:interest
   Expenses:tax (not really needed to show bug)

2) Actions > Scheduled Transactions > Mortgage & Loan Repayment..

3) Enter some arbitrary figures into the wizard.  Make the loan for 6
   months or so, and use a more sensible rate like 1%.  Supply the
   step 1 accounts in the corresponding fields.  Finish the wizard.

4) Actions > Scheduled Transactions > Scheduled Transaction Editor

5) Open the new loan.  Tick "create automatically" and "create in
   advance", and set the advance creation to ~700 days.  That will
   ensure that all transactions are pushed to the register.

6) Save, exit, and start gnucash again to make sure the scheduler runs.

7) Open one of the transactions with the splits showing.

8) Reduce the principle amount by an arbitrary amount.  Before you can
   increase the principle by the same amount, gnucash will force an
   EUR/USD exchange rate dialog even though all accounts are in euros.

Where does USD come from?  Apparently the amortization scheduling tool
assumes USD for something.