APT snapshots: drop i386 architecture

Originally created by @intrigeri on #12605 (Redmine)

This would save us lots of disk space!

At least config/binary_local-hooks/40-include_syslinux_in_ISO_filesystem depends on it though: we could ensure our custom APT repo has the correct i386 version of syslinux, but I’m not sure if it’ll be enough: the apt-get update we run after enabling i386 there might fail given no i386 indices will be available for most APT sources. Worst case, we can simply publish somewhere the i386 /usr/bin/syslinux binary we need, and download+check it without using APT.

I could find no other blocker.

Related issues

• Related to tails/sysadmin#11912
• Blocked by tails/tails#10289
• Blocked by tails/tails#13513
• Blocked by tails/tails#14735