Fix checked states for checkboxes in admin color schemes

[sabernhardt]

Removes CSS for the checked states of both checkboxes and radio buttons from _admin.scss. The white color used in all schemes is already defined in forms.css:

input[type="checkbox"]:checked::before {
	content: url("data:image/svg+xml;utf8,%3Csvg%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%2020%2020%27%3E%3Cpath%20d%3D%27M14.83%204.89l1.34.94-5.81%208.38H9.02L5.78%209.67l1.34-1.25%202.57%202.4z%27%20fill%3D%27%23ffffff%27%2F%3E%3C%2Fsvg%3E");
	content: url("data:image/svg+xml;utf8,%3Csvg%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%2020%2020%27%3E%3Cpath%20d%3D%27M14.83%204.89l1.34.94-5.81%208.38H9.02L5.78%209.67l1.34-1.25%202.57%202.4z%27%20fill%3D%27%23ffffff%27%2F%3E%3C%2Fsvg%3E") / '';
}

Trac 64822

Use of AI Tools: none (by me)

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