Remove unused files (#131)

- Illustrations load from the `content` folders, not from `assets`
- Screenshots load from `static/images`, not from `assets`
- `justimg` shortcode is not used anywhere

`assets/images` and `justimg.html` can be deleted.

Reviewed-on: https://git.selfprivacy.org/SelfPrivacy/selfprivacy.org/pulls/131
Reviewed-by: Inex Code <inex.code@selfprivacy.org>
Co-authored-by: nevfy <yaver4.0.0.0@gmail.com>
Co-committed-by: nevfy <yaver4.0.0.0@gmail.com>

layouts/shortcodes/justimg.html

@@ -1,2 +0,0 @@
-{{ $original := .Page.Resources.GetMatch (printf "**%s*" (.Get 0)) }}
-<img class="card-img-top rounded-img" src="{{ $original.RelPermalink }}">