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 😄 

## 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