Fix inconsistent background color for filter input field

## What does this MR do?
Removes the background color for the filter input field for the [projects dashboard](https://gitlab.com/dashboard/projects) and [groups page](https://gitlab.com/groups/gitlab-org) to stay consistent with the other input fields

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

## Why was this MR needed?
Fixes UI inconsistency

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-29_at_11.07.28_AM](/uploads/dbd02534686a109fc98b838d2f332a42/Screen_Shot_2016-08-29_at_11.07.28_AM.png)

After:
![Screen_Shot_2016-08-29_at_11.07.38_AM](/uploads/4003a622c8568ce9c973a17c8f74557e/Screen_Shot_2016-08-29_at_11.07.38_AM.png)

## 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 #21399

See merge request !6089