Merge branch 'remove-unused-images' into 'master'

Remove unused images

## What does this MR do?
Remove unused images

```
bg-header.png
bg_fallback.png
chosen-sprite.png
diff_note_add.png
icon-search.png
icon_sprite.png
images.png
move.png
progress_bar.gif
slider_handles.png
```

## Are there points in the code the reviewer needs to double check?
Just need to check if the images are still being used 😄

## Why was this MR needed?
To reduce the technical debt of the project

## What are the relevant issue numbers?
Closes #20499 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5601

CHANGELOG

@@ -17,6 +17,7 @@ v 8.11.0 (unreleased)
   - Optimize maximum user access level lookup in loading of notes
   - Add "No one can push" as an option for protected branches. !5081
   - Environments have an url to link to
+  - Remove unused images (ClemMakesApps)
   - Limit git rev-list output count to one in forced push check
   - Clean up unused routes (Josef Strzibny)
   - Add green outline to New Branch button. !5447 (winniehell)