Fix attendee count on event summary report for free events

The 'event summary' report doesn't display the correct number of attendees for free events.

When someone registers through the front end, they appear on the report (because a contribution is recorded).

When you then record someone through the back end they do not appear, because there is no contribution record for the free event.

Steps to recreate:

• Create a free event with online registration
• Register through the front end
• Create another registration through the back end
• View the 'Event Income Summary' report
• Notice that only 1 participant is counted

Fixed by:

Removing the 'group by currency' in the report SQL.

Single events do not support multiple currencies anyway? Ie: You can have one event in dollars, and another event in pounds. But not one event with dollars AND pounds.

PR incoming.