Popover > Unbox header

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### Description

As part of an effort to "unbox" the GitLab UI (:arrow_forward: [watch a video with more context](https://www.youtube.com/watch?v=MxpZuWQH-kk)), the popover component could have a simple update to remove the border between the header and body content and padding. See a similar effort for [unboxing the modal](https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/issues/1221).

**How does this benefit the UI and support unboxing?**

- The popover has hierarchy established with different font weights, the dividing border just reenforces the hierarchy that already exists.
- Similar to the results in the modal, this also allows us to remove the extra padding needed when there are two separate sections, effectively reducing popover height.
- Removing the border between the header and content allows for a more natural reading flow between header and content without the visual break that makes them feel less connected.

![CleanShot_2022-03-02_at_10.21.47_2x](/uploads/30063e66c8421fa282c6b84d4f9099b6/CleanShot_2022-03-02_at_10.21.47_2x.png)

### Result

![CleanShot_2022-03-23_at_13.15.56](/uploads/ff6a16400ca5ca585f95489e61b2f22e/CleanShot_2022-03-23_at_13.15.56.gif)

### Figma file

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### Checklist

Make sure the following are completed before closing the issue:

1. [x] **Assignee**: Design in your working file. When applicable, follow our 
   [structure][structure], [building][building], and [annotation][annotation] guidelines. If you have any questions, reach out to a [Foundations designer][foundations-team].
1. [x] **Assignee**: Update the link to the working file in the issue 
   description.
1. [x] **Assignee**: Ask a [Foundations designer][foundations-team]
   to review your design (ensure they have edit permissions in Figma).
1. [x] **Reviewer**: Review and approve assignee’s design. Specific design 
   questions can be addressed with comments in Figma. Comment in this issue when the content is less specific to the design or requires greater visibility.
1. [ ] **Reviewer**: Assign to a [Figma maintainer][figma-maintainer]
   for final review (make sure they have edit permissions in Figma).
1. [x] **Maintainer**: Review and approve assignee’s changes.
1. [x] **Maintainer**: Merge the branch or add the changes or additions to the
   target file.
     1. [ ] Ensure that all styles and components now belong to the target file.
     1. [ ] For changes to the **Component Library** file, view the components in the Assets panel to ensure they align with our [asset library structure guidelines][structure].
1. [x] **Maintainer**: When applicable, [publish][publishing] any library 
   changes along with a clear commit message.
1. [ ] **Assignee**: If work was not done in a branch (a merged branch will automatically be archived), move your working file 
   to the shared Figma project:
     1. [ ] For Component Library changes, move your file to the [**Component archive**][component-archive] project.
     1. [ ] For all other changes, move your file to the [**Misc archive**][misc-archive] project.
     1. [ ] If you’re a community contributor, please consider [transfering ownership of your draft file](https://help.figma.com/hc/en-us/articles/360040530853) to the maintainer so they can move it to our archive, along with its version history and comments.
1. [x] **Assignee** (or Maintainer, for community contributions): If it’s a new
   pattern or a significant change, add an agenda item to the next UX weekly call to inform the team.
1. [x] **Assignee**: When applicable, [create an MR in this repository][new-gitlab-ui-mr] and apply the "component-guideline" template 
   to create or update the component’s documentation page. If you do not have capacity, [create an issue in this repository using the "Component documentation" issue template][pajamas-component-documentation-template]. Bring the issue to your team planning session for prioritization and scheduling. Mark the new issue as related to this one.
1. [x] **Assignee**: When applicable, [create an issue in GitLab UI using the "Component" issue template][gitlab-ui-component-template] to build or update the component code. Bring the issue to your team planning session for prioritization and scheduling. Mark the new issue as related to this one.
1. [x] **Assignee**: 🎉 Congrats, you made it! You can now close this issue.

[annotation]: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/blob/main/doc/pajamas-ui-kit.md#adding-descriptions-notes-and-annotations
[building]: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/blob/main/doc/pajamas-ui-kit.md#building-components
[foundations-team]: https://about.gitlab.com/company/team/?department=ecosystem-foundations-team
[figma-maintainer]: https://about.gitlab.com/handbook/engineering/projects/#design.gitlab.com
[publishing]: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/blob/main/doc/pajamas-ui-kit.md#publishing-changes
[structure]: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/blob/main/doc/pajamas-ui-kit.md#structure
[component-guideline-template]: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/blob/main/.gitlab/merge_request_templates/component-guideline.md
[component-archive]: https://www.figma.com/files/project/5472112/Component-archive
[misc-archive]: https://www.figma.com/files/project/10620392/Misc-archive
[gitlab-ui-component-template]: https://gitlab.com/gitlab-org/gitlab-ui/-/issues/new?issuable_template=Component
[pajamas-component-documentation-template]: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/issues/new?issuable_template=Component%20documentation
[new-gitlab-ui-mr]: https://gitlab.com/gitlab-org/gitlab-ui/-/merge_requests/new

Story points: 1.0