Query: Remove nextpage block delimiters when setting up global post data.

`WP_Query::setup_postdata()` splits the post up by `<!--nextpage-->`, which causes invalid block data to be contained in the post content.

This change removes the `<!-- wp:nextpage -->` and `<!-- /wp:nextpage -->`, as well.

Props pento, youknowriad, azaozz, noisysocks.

Merges [43940] into trunk.

See #45401.
Built from https://develop.svn.wordpress.org/trunk@44276


git-svn-id: https://core.svn.wordpress.org/trunk@44106 1a063a9b-81f0-0310-95a4-ce76da25c4cd