It will not disappear from your Works in Progress but you will not be able to reclaim it at a later date and have it count towards your Author Challenges total.
The quiz stays in your list and can be accessed as normal-- it just can't be a challenge quiz.
In this case I usually recommend retitling since someone else will be able to claim the title you gave up.
Senior FT Editor (Video Games, Television, Music, and Entertainment)
Chat Board Moderator (Author's Lounge)
Amazing Trivia Race Taskmaster/Commission Hander-Outer/TRICster