If you look at slime-contribs (the default list of contribs to load), it is set to slime-fancy, which includes slime-c-p-c. So that is where the hook to slime-completion-at-point-functions is being added.
You can either eval (slime-c-p-c-unload) after loading SLIME, or customize slime-contribs to load only the contribs you want (recommended by the manual).
-- Vladimir Sedach