[ng] header vars: remove few old redundant variables

[ng] header vars: remove few old redundant variables

* lib/am/header-vars.am (target_triplet, host_triplet, build_triplet):
Remove these, redundant because aliases respectively for the variables
$(target), $(host) and $(build).  See also:
<http://lists.gnu.org/archive/html/automake/2012-07/msg00058.html>
* lib/am/dejagnu.am: Adjust.
* NG-NEWS: Update.
* doc/automake-ng.texi: Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>