[ATMOSPHERE-508] Disable CephPGImbalance (#2022)

Depends-On: #2023
diff --git a/roles/kube_prometheus_stack/files/jsonnet/mixins.libsonnet b/roles/kube_prometheus_stack/files/jsonnet/mixins.libsonnet
index 7c8c45e..2b876b7 100644
--- a/roles/kube_prometheus_stack/files/jsonnet/mixins.libsonnet
+++ b/roles/kube_prometheus_stack/files/jsonnet/mixins.libsonnet
@@ -12,6 +12,10 @@
   // Superseded by CephHealthDetail* alerts
   'CephHealthWarning',
   'CephHealthError',
+
+  // * Dropped `CephPGImbalance`
+  // the balancer module takes care of this
+  'CephPGImbalance',
 ];
 
 // NOTE(mnaser): This is the default mapping for severities: