efi: libstub: Remove zboot signing from build options

The zboot decompressor series introduced a feature to sign the PE/COFF
kernel image for secure boot as part of the kernel build. This was
necessary because there are actually two images that need to be signed:
the kernel with the EFI stub attached, and the decompressor application.

This is a bit of a burden, because it means that the images must be
signed on the the same system that performs the build, and this is not
realistic for distros.

During the next cycle, we will introduce changes to the zboot code so
that the inner image no longer needs to be signed. This means that the
outer PE/COFF image can be handled as usual, and be signed later in the
release process.

Let's remove the associated Kconfig options now so that they don't end
up in a LTS release while already being deprecated.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>