Hide 'Database Upgrade Required' on admin/network/upgrade.php when you are not being asked to upgrade, to prevent confusion.

Merges [25191] to the 3.6 branch.

props duck_.
fixes #24845.



git-svn-id: https://develop.svn.wordpress.org/branches/3.6@25192 602fd350-edb4-49c9-b593-d223f7449a82