Unwanted completion function

I would like to use the default value for the customization setting Slime Completion At Point Functions, (slime-filename-completion slime-simple-completion-at-point). But something has added slime-c-p-c-completion-at-point to the list of functions from outside Customize. I have not altered slime-completion-at-point-functions in my .emacs or anywhere else.

If I use Customize to erase the customization, then I get the desired value for the current session. But there is no option to save the setting and my .emacs is unaltered; in my next Emacs session, slime-c-p-c-completion-at-point is back in the setting.

How can I prevent slime-c-p-c-completion-at-point from being added?