Karabiner-Elements provides built-in uninstaller.
Use the uninstaller to remove Karabiner-Elements from your system.
You can launch the uninstaller from “Uninstall Karabiner-Elements” button on Misc tab.
If you don’t want to use above GUI, you can also uninstall Karabiner-Elements from command line.
sudo '/Library/Application Support/org.pqrs/Karabiner-Elements/uninstall_core.sh'
(Administrator password is required to run the above command.)