As previously discussed in #38229. To recap: `badcert.jpg` was made
unused in 8422dac811 and `tumbeast.png`
and its associated file of `not-found.html` were superseded at some
point by a message in `neterror.html` (if it were still used, it'd
surely be
[embedded](https://github.com/servo/servo/blob/main/components/shared/embedder/resources.rs).)
This PR also removes two unnecessary spaces inside of the new tab's CSS
file, ensuring a consistent 2 space indentation in the file and saving 2
bytes.
Testing: Unnecessary, as this commit only removes unused assets (and
fixes one style nit).
Signed-off-by: maple! <averyrudelphe@gmail.com>
