Edit /etc/default/grub and add:
GRUB_DEFAULT=0
GRUB_TIMEOUT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_DISABLE_OS_PROBER=true
GRUB_RECORDFAIL_TIMEOUT=0
GRUB_BACKGROUND=""
NOTE: Remember to leave GRUB_DISTRIBUTOR and GRUB_CMDLINE_LINUX_DEFAULT alone. Check for any duplicates.
Then run update-grub and reboot.