BREAKING CHANGE: If `beancount-electric-currency` is non-nil, the
currency is appended to transactions, falling back to the ledger's
"operating_currency" option, even in cases where a currency was not
explicitly needed (e.g. only one currency specified for an account).
After receiving input from beancount-mode's author, I reconsidered its
utility and decided to remove it.
Ref beancount/beancount-mode#19