Fixing spacing issue for `code` inside `pre`

Regular (one-line) code inside a `code` tag has some padding around it. This leads to some weird spacing when dealing with code blocks (e.g. inside `pre`).

Removing the horizontal paddings for these. (Also replacing `border: none;` with `border: 0;` because it's shorter)
Merge pull request #2383 from kleinfreund/patch-1