Revert "Change markup for diff insert/delete lines"
This reverts commit b75d2f0e03317c9b37dfdb702f3a959555309890.
This commit has some issues:
- It uses <ins>/<del> not for the content that was actually
added/removed (i.e. the diff), but for the line stats. I don't think
that helps accessibility, it might actually make it worse.
- It adds some CSS to apply text decoration to <ins>/<del>, but only
inside the `diff` class. However, the elements never occur inside the
`diff` class (see first item). The CSS is never used.
- Instead, the place where it was used (line stats) got unintended text
decoration applied, causing a follow-up commit which fixed this the
wrong way, causing yet more issues (fixed in next commit).
I think the one place where it would make sense to apply <ins>/<del> is
the diff itself, but that might be a more serious operation.