Move cleanup of old auto-drafts to wp_cron

[prettyboymp]

get_default_post_to_edit() currently runs a query to delete old auto-draft posts. This doesn't need to be run every time a new post is created and should be converted to a cron task that runs once a day at most.

[scribu]

Maybe this could be tied to the trash emptying job?

[solarissmoke]

Delete old auto-drafts in wp_scheduled_delete instead of get_default_post_to_edit

[scribu]

Marking as dup of #19663 since its patch has already been reviewed.