I had this very same question one week ago. After a long google search session without answers I posted a note in the stack overflow forum (http://stackoverflow.com/questions/8290982/flex-rds-server-license).
As far as I could figure out this jar is part of Flash Builder IDE. It's binary cames compiled with BlazeDS but it's not part of BlazeDS. I think it's not open source at all.
You do only need it in developing time - I saw some hints on internet saying to remove it when going to production to avoid security issues.
I hope it helps.