Remove duplicated deprecation log entry

Author: alexcams

lib/logstash/outputs/elasticsearch.rb

@@ -259,7 +259,6 @@ def register
       log_message = "'failure_type_logging_whitelist' is deprecated and in a future version of Elasticsearch " +
         "output plugin will be removed, please use 'silence_errors_in_log' instead."
       @deprecation_logger.deprecated log_message
-      @logger.warn log_message
       @silence_errors_in_log = silence_errors_in_log | failure_type_logging_whitelist
     end