Set which-func-cleanup-function to fix mode line display

Without this, `which-function` in org buffers returns a string with many text properties, in particular the `display` property, which causes the name of the current heading in to be rendered incorrectly in the mode line. I didn't bother to implement something "undoing" this when disabling the mode, since it's not likely to be robust in the face of anything else that sets which-func-cleanup-function (although then again, neither is enabling it in the minor mode with setq-local).---Happy to add this if you prefer I do so.