Remove overlay background color (obsolete due to Gutenberg PR 15974)

[frontdevde]

BLOCKED: We need to wait for the Gutenberg change to be available on WPCOM

Changes proposed in this Pull Request

• This PR removes the custom background color on the modal overlay. This custom background color became obsolete thanks to the Gutenberg PR Try: Make the modal overlay scrim darker WordPress/gutenberg#15974
• For more details see the corresponding issue (1P) Remove Custom Modal overlay CSS in favour of new default #34306

Testing instructions

• Create a new page with the block editor
• The background-color of the overlay should use the Gutenberg default. No custom override should be present.

Fixes #34306

[matticbot]

Test live: https://calypso.live/?branch=update/starter-page-template-overlay-styles

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[kwight]

Simple sites are still on Gutenberg v5.8, as we've been unable to land a stable version since; next attempt to update should be v6.1 around July 10th.

[getdave]

Thanks @kwight!

Ok so @frontdevde let's defer this until iteration 5 or whenever 6.1 lands.

[marekhrabe]

Don't forget we run in core and atomic too. Is there a way to use some condition or classname to detect older versions?

[obenland]

@marekhrabe With 6.1.1 out now, is this ready to be merged?

[marekhrabe]

If we don't intend to add any version check as I've mentioned in the last message, I would get this merged.

[obenland]

We're optimizing for Simple and Atomic only at this point. I think Core not looking optimally is fine for now