You don't need live boot utilities or
boot-repair for this.
Boot into kali, then run
sudo efibootmgr (or just
efibootmgr as root).
It should display a list that should look something like this:
Timeout: 1 seconds
BootOrder: 0000,0001 <there may be other 4-digit numbers on this line>
<... possibly other BootNNNN lines with other stuff...>
The 4-digit numbers may be different, but the general layout should be the same.
BootOrder tells the system firmware the order in which the various boot targets listed on the
BootNNNN lines should be attempted, and on your system, the number referring to Kali is after the one referring to Ubuntu.
Since you want to get rid of the last vestiges of Ubuntu, the next step is simple: look at the
BootNNNN lines, find the 4-digit number on the line that says
Ubuntu. We'll call that number XXXX.
Now, use this command to delete Ubuntu's UEFI boot entry:
sudo efibootmgr -b XXXX -B
XXXX with the actual four-digit number.
That solves your immediate problem: Ubuntu's GRUB will no longer run at boot time. But it still exists on the disk.
Your ESP partition should be mounted into
/boot/efi, and if you have only one disk, that's where Ubuntu's GRUB actually lives. Look into directory
/boot/efi/EFI/: you should find in there one directory for each OS installed on this disk, probably named as just
kali (capitalization may be different).
To thoroughly remove the last vestiges of Ubuntu's bootloader, just remove the whole
sudo rm -r /boot/efi/EFI/ubuntu
Be careful with this command: if you accidentally remove the
kali directory instead, you'll need to reinstall the bootloader with
grub-install /dev/sda or similar command.