Background images: remove lingering "file" prop

[ramonjd]

What?

This PR is a follow up to #61387 - I missed a reference to source in the theme json class in INDIRECT_PROPERTIES_METADATA. Whoops!

The changes were first added in #59454.

The deleted line is not yet in Core, and the changes are already reflected in the latest 6.6 backport PR:

• Background block supports: add support for top-level styles in theme.json wordpress-develop#6482

Why?

Janitorial

How?

DELETE

Testing Instructions

The CI tests should pass.

If you're feeling up to it, you could test that there are no regressions to site wide background images in the side editor/frontend.

Here is some test json!

{
	"$schema": "../../schemas/json/theme.json",
	"version": 2,
	"settings": {
		"appearanceTools": true
	},
	"styles": {
		"background": {
			"backgroundImage": {
				"url": "https://images.pexels.com/photos/22484288/pexels-photo-22484288/free-photo-of-the-circular-stone-terraces-of-the-inca-ruins.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750&dpr=2"
			},
			"backgroundSize": "contain"
		}
	}
}

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Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

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[andrewserong]

Good catch!

✅ site wide background images are still output in the post and site editors
✅ site wide background images are still output on the site front end
✅ if I manually add a source key to the backgroundImage object, it doesn't affect anything (just in case anyone already has a key added there)

LGTM! 🎉