[ja] revert an incorrect fix in mysql-configmap.yaml

content/ja/examples/application/mysql/mysql-configmap.yaml

@@ -9,10 +9,8 @@ data:
     # Apply this config only on the primary.
     [mysqld]
     log-bin
-    datadir=/var/lib/mysql/mysql
   replica.cnf: |
     # Apply this config only on replicas.
     [mysqld]
     super-read-only
-    datadir=/var/lib/mysql/mysql