maint: don't use "union" merge driver for NEWS anymore

maint: don't use "union" merge driver for NEWS anymore

The use of the "union" merge driver to avoid conflicts in NEWS has
proven to be problematic, as it often silently produced mis-merges
(instead of spurious conflicts, but a spurious conflict is better
than a mis-merge).  Also, our new branching policy (see automake
bug#11153 for a description) should greatly mitigate the problem of
merge conflicts in NEWS.  For further discussion, see:
<http://lists.gnu.org/archive/html/automake-patches/2012-04/msg00148.html>

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