'Change event selections' impossible for sold out events

If someone has booked for an event, and chosen a price option that is 'sold out' then it is not possible to remove this price option from their booking because the input box is 'frozen' as a sold out event.

Steps to recreate

• Create an event with a price set:
  • Checkboxes, required
  • Create 2 options with a max capacity of 1
• Register a new event participant for the event, selecting the price option
• Edit the registration > Change selections
• Note that the selection cannot be edited because the field is 'frozen' as sold out

Proposed solution

Preserve the 'Sold out' label for sold out price options, but don't freeze the input: allow it to be removed from the participant's selections.