Gutenberg: Review and remove obsolete styles from Markdown preview

In order to make the elements inside Markdown preview look like the rest of the elements in the Gutenberg editor, we introduced a bunch of arbitrary styles in #27450.

There have been some discussions in the Gutenberg core that these elements will be styled in a more general and reusable way, which will make the styles we introduced unnecessary.

When that time comes, let's remove those styles that we introduced in #27450.

Review issue #27450 and the Gutenberg core discussion about reusable styling to identify the Markdown preview styles introduced there. Once the general Gutenberg styling is available, remove the obsolete styles and verify that Markdown preview elements still match the editor without them.

Written by the indexing model from the issue text.