uninstall os-prober from all machines

os-prober runs as part of update-grub and tries to detected alternative
operating systems (Windows, Solaris, Hurd, Minix etc.). Fortunately we
use none of them, and having the package installed is not only cruft but
also has the nasty side-effect that in the process of detection (= every
time a kernel gets installed), it modprobes a bunch of filesystems (jfs,
btrfs, hfs+, fat32 etc.) that a) could have bugs or even security
vulnerabilities, b) spawn kernel threads (e.g.  jfsCommit) that sit
there and who knows what they do.

Change-Id: Id97f1d35b45d3c3c1e702288d372e52e4d49aa0c