Remove OpenJDK log warning

The following warning message will appear in the logs when using OpenJDK

WARN  [main] ... OpenJDK is not recommended. Please upgrade to the newest Oracle Java release

The above warning dates back to when OpenJDK 6 was released and there were some issues in early releases of this version. The OpenJDK implementation is used as a reference for the OracleJDK which means the implementations are very close. In addition, most users have moved off Java 6 so we can probably remove this warning message.