I’m sorry I don’t seem helpful. It’s generally assumed that if you installed a plugin that you know how to remove it. If you were one of my clients I could do it for you. If you would like for me to do it for you you can subscribe to this service. EDIT: Your problem appears to be beyond the scope of that service. Best of luck to you!
Edit app.yml. Remove the line that includes the retort plugin.
./launcher rebuild app
See Install Plugins in Discourse for more instructions.