Drop the top-level ChangeLog

Drop the generated ChangeLog, for the following reasons:

 * It's not useful, NEWS covers the user-visible changes in a much more
   digestible format, and the git log is available to everyone who wants
   or needs the more fine grained log.
 * The ChangeLog can only be regenerated from a git checkout, which
   makes it hard to remove it when doing distclean.
 * Not removing it at distclean time makes distcheck fail, and working
   around that is more work than the benefit of having ChangeLog at all.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>