amd64: revert all grub changes for xenial

For LP: #1901906, on other releases we ensured that grub-pc is always
installed with shim-signed. However, xenial has different behavior that
does not work the same. This reverts those changes back to what
livecd-rootfs was before any of those changes were made, keeping only
the autoremove packages work.

LP: #1901906

live-build/auto/config

@@ -501,9 +501,9 @@ case $PROJECT in
 					add_package install grub-pc
 					;;
 				amd64)
-					add_package install grub-pc
-					add_package install shim-signed
+					add_package install grub-pc-bin
 					add_package install grub-efi-amd64-signed
+					add_package install shim-signed
 					;;
 				armhf)
 					add_package install flash-kernel u-boot-tools

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

@@ -86,7 +86,7 @@ install_grub() {
             efi_target=arm64-efi
             ;;
         amd64)
-            chroot mountpoint apt-get install -qqy grub-pc shim-signed grub-efi-amd64-signed
+            chroot mountpoint apt-get install -qqy grub-efi-amd64-signed grub-efi-amd64 shim-signed
             grub_modules="multiboot serial usb usb_keyboard"
             efi_target=x86_64-efi
             ;;