Partial Entries Add-on: Notification with PDF attached is cached

[jakejackson1]

Description
When enabling the Partial Entries add-on on a form and setting up a Notification email with a Gravity PDF-attachment the generated PDF isn't removed from disk after the email is sent and this causes caching problems when the form actually does get submitted, as the old version is used.

Step To Reproduce
Steps to reproduce the behavior:

1. Enable Partial Entries on a form
2. Setup a Notification email that is sent on the event "Partial Entries: Saved"
3. Setup a Notification email to be sent on the event "Form Submitted"
4. Setup a PDF to be attached to both Notification
5. In the form create a few fields that are required
6. Preview the form, fill out one of the fields and hit submit.
7. When a validation error occurs, fill the rest of the form out and submit
8. Preview the PDF attachment from both emails sent

Expected behavior
The Form Submission PDF attachment should not use the cached version and instead correctly show all entered information on form submission.