Fix disabling of the link manager on upgrade from 3.4.2 when there are no links.

In the 3.4 branch we bumped the DB version past the version used for disabling links in trunk, preventing it from running. In the future, we must avoid increasing branch DB version bumps to HEAD.

props SergeyBiryukov.
fixes #22626.



git-svn-id: https://core.svn.wordpress.org/trunk@22911 1a063a9b-81f0-0310-95a4-ce76da25c4cd