Events removed from the cart/session do not remove their registrations

[sethshoultes]

Codebase ticket:
https://events.codebasehq.com/projects/event-espresso/tickets/10671

Testing notes:
https://events.codebasehq.com/projects/event-espresso/tickets/10671#update-42035626

This didn't really add much onto the time it took to track down the problem so created a branch with it.

Why delete the additional regs? Because anywhere that pulls the attendees using the session will run into this if you remove an event from the session after already providing reg details and hitting the payment options.

Refreshing the payment overview overwrites the current attendees in the session and so removes the additional registration anyway.

Branch - BUG-10671-remove-regs-from-session

To reproduce add an event to the cart, select a ticket, add your registration details and click to continue to payment.

From there get back to /event-registration/ (doesn't matter how you do it)

Add another event to the cart, you'll have the original and the new one. Select a ticket on the new event and remove the old one.

Continue through, add your details for the new event and continue - http://take.ms/4uCWr

Now on the Payment Overview you'll see both attendees - http://take.ms/Q6uqY

If you continue with the payment the thank you page shows both those events etc.
If you refresh the above page before continuing onto payment it 'fixes' it for you.

Using this branch that additional attendee is removed from the users session when they click the red stop icon to remove the event from the session.