Move List styles to theme.json

[bgardner]

Per WordPress/gutenberg#53602, it should be safe to move this to theme.json:

/* -- List -- */

.is-style-no-disc {
	list-style-type: none;
	padding-left: 0;
}

Per Gutenberg 16.8 RC1 (WordPress/gutenberg#54981) confirmed this is working in theme.json. Will merge once it’s included in WP6.4 Beta or RC.

"core/list": {
	"css": "&.is-style-no-disc {list-style-type: none; padding-left: 0;}",
	"spacing": {
		"padding": {
			"left": "var(--wp--custom--spacing--gap)"
		}
	}
},