[i386] Stop building kernel packages

The upstream Linux/x86 maintainers no longer spend much effort on 32-bit (i386) configurations, while the compat mode of 64-bit (amd64) kernels is well-tested.

As discussed previously with the release team, i386 user-space should continue to be supported through chroots or multiarch on amd64 hosts, but i386 kernel packages should not be provided for future releases.

This removes all configuration for i386 kernel packages, while leaving user-space packages untouched.

In debian/templates/image.bug/include-model, it also removes the last reference to ia64, which was missed in the earlier removal.