Site Editor: remove the default admin CSS

[ellatrix]

See WordPress/gutenberg#66431.

Let's let it sit in Gutenberg for a bit in case there end up being a style regressions.

Trac ticket:

---

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props ellatrix, apermo, wildworks.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[apermo]

Suggested change

	// We do not need the default admin CSS stylesheets in the Site Editor.
	// Do not load the default admin CSS stylesheets if in the Site Editor.

That's a little more into the point imho.

[t-hamano]

It was pointed out in this comment that removing admin CSS from the site editor was problematic.

In Gutenberg, this change has been reverted: WordPress/gutenberg#66540