Delete expired transients on database upgrades.

Reverts [25416], which had all transients being cleared. This leaves much to be desired, but we don't want a core update to be blamed for breaking a site that incorrectly assumes transients aren't transient.

props dartiss, pento.
fixes #20316.


git-svn-id: https://develop.svn.wordpress.org/trunk@25838 602fd350-edb4-49c9-b593-d223f7449a82