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.
Additionally, this will ensure that autoremove is run after installing
anything in the CPC build hooks. This is done to avoid shipping images
that include packages that are autoremovable. This will clean-up as
packages are installed and detect any breakage at build time.
LP: #1901906
