Corrects: discount for an event apply to all pricesets, regadless of user preferences

When creating a discount for an event, you have to select all discountable pricesets on the discount edit screen as per the help text on the screen. But on the event registration page, the discount will apply to all pricesets related to this event, ignoring what you chose on the setup screen. So there is no way to apply the discount to only selected pricesets for an event (ie. discount admission fee but not the lunch for example).

This patch seem to correct the issue, but I had trouble understanding the logic of the original code since it seemed to overwrite the selection of pricesets from the setup screen with all available pricesets for the event on purpose.