Undo bullseye bug circumvention

Undo the circumvention of a problem with Debian Bullseye in combination with Xen.

We can choose to already do this now, because Greenhost fixed it on their machines and the likelihood of someone else running into the problem at a different VPS provider seems very slim, but we can also decide to wait for https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=983357 to be properly closed first.