Drop obsolete multiarch handling in Stretch

We still have i386-related stuff in config/chroot_local-hooks/04-apt-architectures and config/chroot_local-hooks/99-disable-multiarch. In particular, the resulting system is configured with APT::Architectures {"i386";};, which feels wrong.

Feature Branch: bugfix/11961-drop-multiarch

Related issues

Blocks #11873 (closed)

Original created by @intrigeri on 11961 (Redmine)

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