How do I remove Windows from the UEFI boot menu after custom installing Ubuntu?

Find UEFI boot entries

To remove the now useless Windows boot entry in the UEFI, first you need to find it. This can be done directly from the main Ubuntu OS. There is no need to boot from a live USB/DVD. The installed Ubuntu already has the required commandline utility.

Open a terminal by pressing Ctrl+Alt+T and then enter:

sudo efibootmgr

Enter the password when prompted and hit Enter. The cursor won't move and you won't see any asterisks(*****). This is normal in Ubuntu terminal. You will see some output like:

BootCurrent: 0002
Timeout: 2 seconds
BootOrder: 0002,0001,000
Boot0000* Dell
Boot0001* Windows Boot Manager
Boot0002* ubuntu

Note, I got two entries other than windows, 0 for "Dell" and 1 for "Windows Boot Manager". Since I deleted all the original partitions except EFI, I can no longer boot "Dell" or "Windows Boot Manager." I could delete both 0 and 1. But for this answer I will show how to delete only the Windows Boot Manager entry.

Note: the entries you want to delete may be different than this example. Use the output you get from your own computer, not the above example, to decide which entries you want to delete.

Delete the obsolete boot entries

Now that you know you want to delete the entry 1 (in this example), In the terminal enter:

sudo efibootmgr -b 1 -B

Note, the 1 in the command above. This command will delete the entry:

Boot0001* Windows Boot Manager

Delete the corresponding folder from the EFI partition

We are almost done. This last step makes sure the UEFI does not regenerate the entries we just deleted. To do this you have delete a folder corresponding the entry you deleted in the previous steps.

Now you find the name and location of the folder to delete. Use the following command to list all the folders in the EFI subfolder in the partition:

sudo ls /boot/efi/EFI

You will see some output like:

Dell Windows ubuntu

To delete the Windows folder use the following command:

sudo rm -r /boot/efi/EFI/Windows

To verify that the folder was deleted, use the sudo ls /boot/efi/EFI command again.

Finally update GRUB to hide the GRUB menu

At this stage when you boot the laptop, GRUB will still show the menu to choose various OSs, even though you have only Ubuntu installed. Edit the file /etc/default/grub with the following command. You may use any other editor if you want.

sudo nano /etc/default/grub

Make the two lines look like this:

GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=0

If the line GRUB_TIMEOUT_STYLE=hidden does not exist, add the line as above.

When done, save and exit the editor by Ctrl+X, followed by Y and Enter.

The final step is to update GRUB with the following command:

sudo update-grub

Now the laptop should directly boot Ubuntu without showing the GRUB menu. If you need to get to the recovery mode in the future, then hit Esc while the laptop boots to reveal the GRUB menu.

Reference: This answer is based on How do I remove "Ubuntu" in the bios boot menu? (UEFI)