EFI: Purge grub-pc after install grub-efi

grub-efi-amd64 removes grub-pc, but grub-pc leaves residual config files
When the user purges it, debconf asks if /boot/grub should be wiped. If the user says yes (which is not the default option)
the system becomes unbootable.

usr/lib/live-installer/installer.py

@@ -124,6 +124,7 @@ def start_installation(self):
  os.system("cp /run/live/medium/pool/main/g/grub-efi-amd64-signed/* /target/debs/")
  os.system("cp /run/live/medium/pool/main/s/shim*/* /target/debs/")
  self.do_run_in_chroot("dpkg -i /debs/*")
+ self.do_run_in_chroot("DEBIAN_FRONTEND=noninteractive apt-get remove --purge --yes grub-pc")
  os.system("rm -rf /target/debs")
 
  print(" --> Installing microcode packages")