[SharedUX] Remove SCSS from kibana_react (elastic#204497)

## Summary

Part of elastic/kibana-team#1082

Removes all Sass files in `src/plugins/kibana_react` to replace with
styles declared with Emotion. This PR does not include any changes that
would be noticeable by end-users. It changes the internals to use a
different technology for styling components.

## References
1. https://emotion.sh/docs/globals
2. https://emotion.sh/docs/best-practices
3.
elastic/eui#6828 (comment)

src/platform/plugins/shared/kibana_react/public/page_template/index.ts

@@ -7,5 +7,5 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-export { KibanaPageTemplateSolutionNavAvatar, KibanaPageTemplateSolutionNav } from './solution_nav';
+export { KibanaPageTemplateSolutionNavAvatar } from './solution_nav';
 export * from './no_data_page';

src/platform/plugins/shared/kibana_react/public/page_template/solution_nav/index.ts

@@ -7,9 +7,5 @@
  * License v3.0 only", or the "Server Side Public License, v 1".
  */
 
-export type { KibanaPageTemplateSolutionNavProps } from './solution_nav';
-export { KibanaPageTemplateSolutionNav } from './solution_nav';
 export type { KibanaPageTemplateSolutionNavAvatarProps } from './solution_nav_avatar';
 export { KibanaPageTemplateSolutionNavAvatar } from './solution_nav_avatar';
-export type { KibanaPageTemplateSolutionNavCollapseButtonProps } from './solution_nav_collapse_button';
-export { KibanaPageTemplateSolutionNavCollapseButton } from './solution_nav_collapse_button';