FAQ: Delete section about Ubuntu 10.04.

The link was broken, and we don't support Ubuntu 10.04 (maybe the
'oldlinux' branch does).
FAQ: Move #debug anchor back to the right place.

This was accidentally moved by this commit:

  commit b8b5ed65c26dd19a0bc9f8282a50c6dff90c456a
  Author: Richard W.M. Jones <rjones@redhat.com>
  Date:   Wed May 23 11:46:23 2012 +0100

    FAQ: Add section about using libguestfs in closed source programs.
FAQ: Replace the debugging section (again) with a checklist of information we need to start to fix bugs.