I have the same in my install script for my system, for those curious, or those coming here from google:
sudo cp /etc/default/grub /etc/default/grub.bak # Make backup of grub.
sudo sed -i '/GRUB_CMDLINE_LINUX_DEFAULT=/c\GRUB_CMDLINE_LINUX_DEFAULT=""' /etc/default/grub # Remove the quiet boot.
sudo update-grub # Update the grub.
For the command ‘update-grub’ to work I have installed the package ‘update-grub’. It is basically a wrapper for the commands earlier in the thread.