Unwanted GRUB on macOS High Sierra
Assuming you did not create any new partitions on your Mac, then Grub will only exist in the hidden EFI partition. If there is no evidence of Grub in the Startup Manager menu, then you can just leave the Grub files on the EFI partition. 

You can make macOS the default by holding down the control key before selecting macOS in the Startup Manager menu.

If Grub does appear in the Startup Manager, then you can fix this by removing the EFI/BOOT folder from the hidden EFI volume. To make this volume visible in the Finder, execute the following command.

sudo diskutil mount disk0s1