Admin: Fix checked input state in older browsers.

Remove duplicate CSS setting the checked state for checkbox and radio inputs from `_admin.scss`. The duplicate selectors overrode the white checkmark in older browsers, impacting alternate color schemes.

Props sabernhardt, joedolson.
Fixes #64822.

git-svn-id: https://develop.svn.wordpress.org/trunk@61940 602fd350-edb4-49c9-b593-d223f7449a82