[SharedUX/SCSS] remove scss from selected components (#207008)

## Summary
Part of https://github.com/elastic/kibana-team/issues/1417

This PR converts a batch of SCSS to Emotion in the SharedUX domain.

* `KibanaSolutionAvatar`
* `FilePicker`
* `SolutionNav`
* `SolutionNavCollapseButton`
* `KbnTopNav`

All of the changes, except for `KbnTopNav`, can be tested in Storybook
by running `yarn storybook shared_ux`

### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

### Identify risks

Does this PR introduce any risks? For example, consider risks like hard
to test bugs, performance regression, potential of data loss.

Describe the risk, its severity, and mitigation for each identified
risk. Invite stakeholders and evaluate how to proceed before merging.

- [x] The risk of inexact conversion: verifying this PR requires manual
checks to ensure that the conversion has not created any regressions in
the style.

examples/screenshot_mode_example/public/index.ts

@@ -7,8 +7,6 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-import './index.scss';
-
 import { ScreenshotModeExamplePlugin } from './plugin';
 
 // This exports static code and TypeScript types,