Refactor .scss files import to one place

This approach is used in the Gutenberg repository.
Our original approach, where we imported styles in index files in subdirectories, was a bit fragile
because such a file might be skipped in case a component is imported directly.

[MAILPOET-6320]

packages/js/email-editor/src/components/header/index.ts

@@ -1,3 +1 @@
-import './index.scss';
-
 export * from './header';