SCSS - don't overwrite default functions

[cscheid]

See #10529 (reply in thread) for context.

(cc @gadenbuie)

The problem is that it's hard to tell if anyone is relying on this function.

That's fair, but I don't think we declare this function as part of Quarto's public API. The real problem is that Quarto doesn't (and will never) have a sharply defined API boundary to begin with. But, if anything, that argues for making such fixes sooner rather than later.