amd64: always install grub-pc with shim-signed

shim-signed depends on grub-efi-amd64-signed, which in turn has
alternative depends on either `grub-efi-amd64 | grub-pc`. However to
support booting with either via shim&signed-grub and BIOS, the choice
must be made to install grub-pc, not grub-efi-amd64.

This makes images consistent with Ubuntu Deskop, Live Server, buildd
bootable images; all of which already do install grub-pc and
shim-signed.

LP: #1901906

live-build/auto/config

@@ -778,8 +778,7 @@ case $PROJECT in
 				add_package install grub-pc
 				;;
 			amd64)
-				add_package install grub-pc-bin
-				add_package install grub-efi-amd64-signed
+				add_package install grub-pc
 				add_package install shim-signed
 				;;
 		esac

live-build/ubuntu-cpc/hooks.d/base/disk-image-uefi.binary

@@ -89,7 +89,7 @@ install_grub() {
             efi_target=arm-efi
             ;;
         amd64)
-            chroot mountpoint apt-get install -qqy shim-signed
+            chroot mountpoint apt-get install -qqy grub-pc shim-signed
             efi_target=x86_64-efi
             ;;
     esac