Once your system has been upgraded to Ansible Tower 3.0, you will find that your system tracking data has been migrated from MongoDB to PostgreSQL. If you want to delete the old data in MongoDB, you can do so manually. First, connect to your mongo database using the mongo command line client, then run the following commands:
$ use system_tracking
$ db.runCommand( { dropDatabase: 1 } )
At this point, you can also remove the MongoDB packages.