The warning only occurs on EM startup, when the em.log records all messages directed to standard output (stdout).
This is an example of logged messages due to the use of third party code used in the EM. The message is harmless and can be ignored.
As a general point, any message with no timestamp is often due to Java coding and the use of deprecated functionality. The compiler will complain with some warnings but the program still runs fine. As a result, the user can ignore warnings about deprecated functions, unless there is a problem that can specifically be attributed to the message.