Explain that we need updates from distribution kernel for security and
for nxd-fuse.ko;

Remove stable-proposed-updates as we no longer need it.
Stable-proposed-updates was needed once in c02d3ea1 (playbook: Use
proposed updates for debian9 to get an even newer kernel) to install
4.9.185 when it was present only in there and not in stable. Those days
are over and all my general kernel fixes that are not localized in FUSE
module[1,2,3] are now shipped by standard distribution kernels back to
3.16.x on Debian 8 and 4.4.x on Ubuntu 16.04 LTS.

Suggested by @tomo:

nexedi/slapos.package!133 (comment 120609)

[1] git.kernel.org/linus/10dce8af3422
[2] git.kernel.org/linus/c5bf68fe0c86
[3] git.kernel.org/linus/438ab720c675