I wouldn't eat it. Celery is supposed to stay a little crisp even when it's been cooked a bit. Soggy celery sounds like the moisture in the stalk is leaving and collecting on the outside. The only thing I can think of other than salt is old age or some other chemical reaction. Celery is cheap. Chuck it and get some fresh. Also, how was it stored? Was it in an air tight bag, or loosely wrapped and what type of bag material? I keep picturing rotten soggy celery in a plastic zip lock bag. Doesn't sound very tasty at all.