Thank you for the post.
To remove the stdrr log file, first stop the FileMaker Server. Reference the FileMaker Server 12 Help Guide for the instructions specific to the operating system in use:
Starting and stopping FileMaker Server is on page 32.
Starting or stopping the FileMaker Server service (Windows) is on page 223.
Starting or stopping FileMaker Server background processes (Mac OS) is on page 224.
Once the server and corresponding services are stopped, the log file can be deleted since the file will no longer be in use.