Don't include "Pay Later" text in non-paid events

To reproduce:
1. Create an event that is a paid event with "Pay Later" option enabled. Add some text to the Pay Later Instructions field. Save it.
2. Edit the event, setting the "Paid Event" radio button to "No". Save it.
3. Register for the event.

What happens: the Pay Later Instructions appear in the receipt email even though the event isn't a paid event anymore.

Workaround: Edit the event, click "Yes" in the "Paid Event" radio button so that the form appears, uncheck the "Pay Later" checkbox, and then click the "No" radio button in Paid Event. Save the event.

I don't know if this issue is specific to Events, or if the Pay Later Instructions text shows up in other places that it shouldn't.