Hi Angel,
In order to mass delete you can utilize Bulk Delete jobs inside of the CRM system.
To locate this, go to Settings - Data Management - Bulk Record Deletion.
From here you will be able to create a Bulk Delete job, specify the Note entity, and add the "Regarding" filter to look at Case.
Here is a screenshot of the query:

This will delete all Notes that are regarding a Case.
Once you hit Next and progress through the wizard, the Bulk Delete job will run and the notes will be deleted in the background.
If this solved your issue please verify this post, and if you have any other questions just let me know!
Thanks!
Matt Bayes