Revert inclusion of syslinux:amd64 in the ISO.

It was introduced (#9381) for reasons that ended up being wrong (#9594).

Here, we also remove amd64 APT sources and dpkg's support for amd64 as a foreign
architecture. We'll need them again when we want to ship Linux 4.x, but once
we're there we can perhaps enable amd64 sources only for selected APT
repositories, to avoid re-introducing #9381.

Reverts:
e9d2e345
181c6d26
e1d331aa

Will-fix: #9748