Fix PrometheusRemoteWriteDesiredShards

This rule has the same labels on both sides. We don't want
`group_right` and `on`, we want nothing.
Signed-off-by: Nbeorn7 <beorn@grafana.com>

documentation/prometheus-mixin/alerts.libsonnet

@@ -218,7 +218,7 @@
               # https://www.robustperception.io/alerting-on-gauges-in-prometheus-2-0 for details.
               (
                 max_over_time(prometheus_remote_storage_shards_desired{%(prometheusSelector)s}[5m])
-              > on(job, instance) group_right
+              >
                 max_over_time(prometheus_remote_storage_shards_max{%(prometheusSelector)s}[5m])
               )
             ||| % $._config,