Suggest removal of xkeystone

x11-xserver-utils includes the file /usr/bin/xkeystone

xkeystone is a nickle script, and x11-xserver-utils does not depend on nickle, and even if you install nickle the script fails with a massive amount of syntax errors. (as tested with both bionic and focal packages).

It's not clear to me what xkeystone is supposed to do - there's no man page or any documentation I could find. The only thing I was able to find are various threads where other distros remove this binary, so I suggest we remove it from ubuntu as well.