openmpi-nightly-tarball.sh: don't even check v1.6 any more

The create_tarball.sh in v1.6 assumes SVN.  So let's not check that
any more...

contrib/build-server/openmpi-nightly-tarball.sh

@@ -26,8 +26,8 @@ script_dir=/u/mpiteam/scripts
 
 # The tarballs to make
 if [ $# -eq 0 ] ; then
-    # We're no longer ever checking the 1.0 - 1.4 branches anymore
-    branches="master v1.8 v1.6"
+    # We're no longer ever checking the 1.0 - 1.6 branches anymore
+    branches="master v1.8"
 else
     branches=$@
 fi