Removed unused method in DeprecatedKeyInfo to solve spotbugs warning

Author: zabetak

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/conf/Configuration.java

@@ -379,10 +379,6 @@ private static class DeprecatedKeyInfo {
       this.customMessage = customMessage;
     }
 
-    private final String getWarningMessage(String key) {
-      return getWarningMessage(key, null);
-    }
-
     /**
      * Method to provide the warning message. It gives the custom message if
      * non-null, and default message otherwise.