In the Process Builder, it is possible for one to delete the existing process under the same package using the option available in the process's properties.
Figure 1: Deleting Process
However, if you wish to delete every single process you will need to go through these steps instead:-
Export the app to get the jwa file.
Open the jwa file and delete the package.xpdl file.
Import the app back into Joget and you will notice that there are no more process designs.