Remove inconsistent font weight for sidebar's labels

## What does this MR do?
Removes the `.bold` css class from the sidebar label default value so that it is no longer bold 😄 

**None** => None

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

## Why was this MR needed?
Fixes a UI inconsistency

## What are the relevant issue numbers?
Closes #21141 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-24_at_9.41.11_AM](/uploads/3e416d45531b7322474922c569225c03/Screen_Shot_2016-08-24_at_9.41.11_AM.png)

After:
![Screen_Shot_2016-08-24_at_9.40.57_AM](/uploads/a4d60d4cd0a57142ad6c0c8a25be5342/Screen_Shot_2016-08-24_at_9.40.57_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)

See merge request !5984