Remove Yahoo quote sources

Removed the references to Yahoo quote sources so that they will not be
created anymore automatically.
Renamed the standard currency exchange rate source to 'KMyMoney
Currency' and assigned it to fx-rate.net until this is user selectable.

Patch recreated from commit 8559f9ec

BUG:386462
FIXED-IN:4.8.2
Differential Revision: https://phabricator.kde.org/D8715