Replace PDF cleanup after form submission

[jakejackson1]

Description

Background processing emails is becoming more common, and auto-deleting the PDF attachments right after form submission can cause errors when the async email is sent. To correct this, replace the auto-cleanup logic with a scheduled event that runs 1 hour after the initial form submission.

WP SMTP Mail v4 includes async mail sending and it should be possible to replicate the error with it.

[jakejackson1]

Reviewed WP SMTP Mail and found it copies the file attachments to a tmp location so the file will always be present when the queue is run. It's not the cause of the issue.