Removing event selection from participant does not free up capacity

If you change event selections on a participant to remove them from a price option, it doesn't free up space on the option for someone else to book.

Steps to recreate:

• Add a price option checkbox with 'max participants' of 1 to a new event
• Register someone to the event, selecting the option
• Go to the front-end form: notice it says the option is sold out
• Now go to the booked-in person's participant record
• Change their selections to 'remove' the price option, save
• Back back to the front-end form: it still says the option is 'sold out' even though it isn't