Fix Issues with DataProduct Deletion in DeleteView

[davner]

This pull request addresses two issues related to the deletion of DataProduct objects. The first issue was that the files were not being properly deleted, causing name collisions for newly added files. This is in context of when adding previous observational data by obs ID.

In the previous version, deleting a DataProduct was being ignored because:

DeleteView uses FormMixin to handle POST requests. 
As a consequence, any custom deletion logic in DataProductDeleteView.delete() handler 
should be moved to form_valid().

This problem was noticed when a random string was being appended to filenames, due to collisions caused by the delete() method being ignored, after being deleted and re-added. Django appends such a string to avoid filename collision when the file is not actually deleted.

The second issue was that the DataProduct reference was not being removed, leading to errors in subsequent calls to creating data product context. This was only realized after moving to form_valid().

Changes:

• Moved the deletion logic from DataProductDeleteView.delete() to form_valid() to comply with Django's DeleteView behavior. This ensures the associated file is actually deleted.
• Added a call to data_product.delete() to remove the DataProduct reference from the database, resolving the None reference issue.

This image shows how the filename was changed after deleting and re-adding.

I am using Django 4.2.5

[jchate6]

Straight from the Django 4.0 Documentation

[jchate6]

Most beautiful PR I've seen in ages.
Thanks for this.