Strip the display posts shortcode from content before rendering

If one post uses the shortcode to load another post, which then also uses the shortcode to include the original post, PHP ends up in an infinite loop. We've fixed this by stripping the shortcode from the 2nd post (if it exists).

The fix is in https://github.com/Automattic/display-posts-shortcode/commit/00b87e8d24509838c2f3b4fa5c55e4cabb3bcb40 but it looks like this pull request is picking up a bunch of mess from previous commits (i.e. this one https://github.com/Automattic/display-posts-shortcode/commit/cc41257f4348ba185107ca5b28575bf72eb29920). Let me know if you want a clean pull. :)