Check if background core updates are supported on about.php.

see #25603, #22704.


git-svn-id: https://develop.svn.wordpress.org/trunk@25833 602fd350-edb4-49c9-b593-d223f7449a82
Remove redundant code. see #25603, #22704, [25833].


git-svn-id: https://develop.svn.wordpress.org/trunk@25834 602fd350-edb4-49c9-b593-d223f7449a82