Merge branch 'remove-hex-shorties' into 'master'

Remove SCSS rules for short hex chars.

## What does this MR do?
Remove CSS hex shorthands linter
## Are there points in the code the reviewer needs to double check?
That I did it right. 
## Why was this MR needed?
To remove SCSS linter for hex shorthands
## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/22920

cc @connorshea

See merge request !6663

.csscomb.json

@@ -6,7 +6,7 @@
   "always-semicolon": true,
   "color-case": "lower",
   "block-indent": "  ",
-  "color-shorthand": true,
+  "color-shorthand": false,
   "element-case": "lower",
   "space-before-colon": "",
   "space-after-colon": " ",

.scss-lint.yml

@@ -79,7 +79,7 @@ linters:
   
   # HEX colors should use three-character values where possible.
   HexLength:
-    enabled: true
+    enabled: false
   
   # HEX color values should use lower-case colors to differentiate between
   # letters and numbers, e.g. `#E3E3E3` vs. `#e3e3e3`.