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

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

Merges [25893] to the 3.7 branch.
fixes #25682.

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git-svn-id: http://core.svn.wordpress.org/branches/3.7@25806 1a063a9b-81f0-0310-95a4-ce76da25c4cd