Make the Recent Comments dashboard widget more performant on sites with large...

Make the Recent Comments dashboard widget more performant on sites with large amounts of comments, in particular with a heavy spam ratio.

Simplifies the query by avoiding a join, and leverages the API now rather than a direct query.

fixes #14222.



git-svn-id: https://core.svn.wordpress.org/trunk@20609 1a063a9b-81f0-0310-95a4-ce76da25c4cd