Merge branch 'remove-redundant-mixins' into 'master'

Remove redundant mixins

## What does this MR do?
Removes the following redundant mixins
```
@mixin box-shadow($shadow) {
  box-shadow: $shadow;
}

@mixin border-radius($radius) {
  border-radius: $radius;
}
```

## Are there points in the code the reviewer needs to double check?
Just need to make sure the refactor didn't break anything. Last time I did something like this, we faced an edge case where values `.5` would pass correctly in mixins but not when passed directly to the property.

## Why was this MR needed?

*  Reduces redundant code which leads to happier developers :smile: 

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [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)

## What are the relevant issue numbers?
Closes #22012

See merge request !6287

1 files changed, 1 insertions, 0 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 24f77442f1a..c848aa1017b 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -36,6 +36,7 @@ v 8.13.0 (unreleased)
   - Add word-wrap to issue title on issue and milestone boards (ClemMakesApps)
   - Fix todos page mobile viewport layout (ClemMakesApps)
   - Fix inconsistent highlighting of already selected activity nav-links (ClemMakesApps)
+  - Remove redundant mixins (ClemMakesApps)
   - Fix robots.txt disallowing access to groups starting with "s" (Matt Harrison)
   - Close open merge request without source project (Katarzyna Kobierska Ula Budziszewska)
   - Fix that manual jobs would no longer block jobs in the next stage. !6604