Confirm delete_site_option() exists before calling it during an update.

This function won't yet exist when updating from 2.7 [sic].

fixes #25682 for trunk.

Built from https://develop.svn.wordpress.org/trunk@25893


git-svn-id: http://core.svn.wordpress.org/trunk@25805 1a063a9b-81f0-0310-95a4-ce76da25c4cd