By default, both ISOLINUX (for BIOS boot) and GRUB 2 (for EFI boot) are installed into the images. A loopback.cfg file is also installed to allow for the image to be booted while still an ISO image in a filesystem.
You can pass --no-isolinux to prevent the installation of ISOLINUX to the image if you do not want ISOLINUX.
You can pass --no-grub to prevent the installation of GRUB to the image.
If you do not want GRUB installed but you would still like the loopback.cfg file to be installed, you can pass --grub-loopback-only.