Remove CurrencyConverterInterface::loadAll()

Not all converters are able to get all rates they offer at once. Yahoo! Finance, for instance, offers 152 currencies, which would result in 152^2 = 23104 individual requests to the Yahoo! server, because the API does not support retrieving multiple rates with the same request. #1905728: Fill {currency_conversion_rate_db_table} in a different way already deals with removing the only dependency on this method, so when that issue is fixed, we can remote the methods themselves.