Sitemaps: Do not unnecessarily call `WP_Query::get_posts()` in posts provider.

The posts have already been fetched at this point, no need to do it again.

Props Chouby.
Fixes #50463.
Built from https://develop.svn.wordpress.org/trunk@48282


git-svn-id: https://core.svn.wordpress.org/trunk@48051 1a063a9b-81f0-0310-95a4-ce76da25c4cd