preferred-xorg-versions: remove include files for old releases

preferred-xorg-versions: remove include files for old releases

* git grep shows that all distributions (in OE tree) moved to
  preferred-xorg-versions-X11R7.5.inc already
* would be nice to encourage everybody to use most tested X11R7.5 or
  no preferred-xorg-versions at all (we can keep latest versions working
  and use D_P -1 for stuff which needs more testing first)
* replace preferred-xorg-versions.inc with link always adjusted to
  latest relase (is there any problem with symlink in OE tree?)
* this would also make easier to remove really old versions when it's
  hard to test some changes (like converting libx11 staging)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Acked-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>