MariaDB: Remove unused exporter password credentials

This removes auth credentials from the mariadb exporter endpoint,
as the credentials used for the actual user are consumed from
the oslo_db endpoint instead.

Change-Id: I1419f50b7e0059763588028b55c982ad83446a33

mariadb/values.yaml

@@ -224,10 +224,6 @@ endpoints:
         public: 80
   prometheus_mysql_exporter:
     namespace: null
-    auth:
-      user:
-        username: exporter
-        password: password
     hosts:
       default: mysql-exporter
     host_fqdn_override: