Fix a couple javac warnings in NFS

This JIRA is to fix 2 javac warnings, which are overlooked in HADOOP-11195.

{format}
241a242,243
> [WARNING] /home/jenkins/jenkins-slave/workspace/PreCommit-HADOOP-Build/hadoop-hdfs-project/hadoop-hdfs-nfs/src/main/java/org/apache/hadoop/hdfs/nfs/conf/NfsConfiguration.java:[46,41] [deprecation] NFS_USERGROUP_UPDATE_MILLIS_KEY in org.apache.hadoop.nfs.nfs3.Nfs3Constant has been deprecated
> [WARNING] /home/jenkins/jenkins-slave/workspace/PreCommit-HADOOP-Build/hadoop-hdfs-project/hadoop-hdfs-nfs/src/main/java/org/apache/hadoop/hdfs/nfs/conf/NfsConfiguration.java:[48,41] [deprecation] NFS_STATIC_MAPPING_FILE_KEY in org.apache.hadoop.nfs.nfs3.Nfs3Constant has been deprecated{format}