xorg: upgrade to X11R7.6 and remove preferred-xorg-versions*.inc files

xorg: upgrade to X11R7.6 and remove preferred-xorg-versions*.inc files

* X11R7.6-RC1 was already released
  http://www.x.org/releases/X11R7.6-RC1/doc/xorg-docs/ReleaseNotes.html
  so it's IMHO time to start migration there
* preferred-xorg-versions-live.inc was tracking latest stable released
  tarballs, but only reason I was maintaining it was to overwrite older
  preferred-xorg-versions-X11R7.5.inc if your distribution included ie
  X11R7.5.inc and you wanted live.inc from local.conf
* default recipe versions without any preferred-xorg-versions include
  are now the same as live.inc
* we will get better test coverage if we use same versions
* if someone really needs older release then pinning only core stuff
  like xserver/libx11 will be probably better, than pinning every xorg
  app/proto/font etc.
* I can slow down adding newest xorg stuff or ie keep it everytime for
  at least a week on ML for another review and test on more targets
  (I'm testing it only on om-gta02/spitz/nokia900)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Acked-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Ackey-by: Tom Rini <tom_rini@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>