Fix and enable unused-variable lint

In _server.py start_server_batch_result is removed because
start_server_batch can only ever fail as a result of a programming
defect in gRPC Python and not the application. This differs from some
analogous-appearing points in _channel.py where the result of
start_client_batch is checked because at those points it is possible
for a failure to indicate a programming defect on the part of the
application.