Remove links_recently_updated_time (hardening).

Merges some of [27916] to the 3.8 branch.

props Tom Adams.
see #27649.

Built from https://develop.svn.wordpress.org/branches/3.8@27917


git-svn-id: http://core.svn.wordpress.org/branches/3.8@27747 1a063a9b-81f0-0310-95a4-ce76da25c4cd