When you have a single boot system but GRUB still shows up, you probably want to hide it, since it adds to your boot time. Here is how 😃

First you will need to install the grub-customizer.

sudo apt install grub-customizer

Next you just have to start it and go to General Settings to disable the options Hide Menu and Look for other Operating Systems. See screenshot below.

Press Save and you're done! 👌