Another way to deal with that is patching the running Lisp and "undefine" no longer needed functions, variables, classes, ...

I have always found that a weak point. Is there an "opposite" key binding to C-c C-c than will undefine/unintern an entity?

C-c C-u is bound to `slime-undefine-function`, though it prompts you for the name of the function and doesn't determine it by the present context. `slime-unintern-symbol` is also present but not bound by default and also prompts for the name.

Can't remember what SLIME supports, but LispWorks has Undefine as an Editor command and in its Editor menus. It can undefine more than just functions.