Remove vendor prefixes for linear-gradient CSS

## What does this MR do?
Removes the vendor prefixes for linear-gradient and the linear-gradient mixin since they are no longer needed with our latest and greatest browser (as per [annabeldunstone](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5762#note_14353141))  

## Are there points in the code the reviewer needs to double check?
None

## Why was this MR needed?
Reduces unnecessary CSS

## 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 [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 #21558

See merge request !6063