Make neutron-grenade non-voting on stable/train

The stable/ussuri, stable/train, and stable/stein branches are
in Extended Maintenance mode and no longer released from, so make
neutron-grenade non-voting in check and remove it from the
gate.

All other grenade jobs are already non-voting in stable/train and
were removed from the gate four years ago by change I82f539bfa533.

Change-Id: I21975b270786687149b232866b2009c145ad9915
Related-bug: #1968798