revealjs a11y: line numbers in code cells

[cscheid]

reprex:

---
format: revealjs
---

```{python}
#| echo: true
print("hello")
a = 3
```

The line numbers are all links with no content. That's a no-no.

[cwickham]

Just realized my recently opened #14655 is a duplicate of this. Closing this in favor of that since it has a bit more detail of the actual violation. I'll note the revealjs part there.