I had earlier done some scripting that broke my report, and it was very hard to debug. Fortunately, I found it.
However, it got me wondering, how do I completely delete a BIRT report associated with a workflow and start fresh with everything inside the report reset, just like when I first opened the report for that workflow?
In the top level directory of the workflow, there are two files:
default_report.rptconfig
default_report.rptdesign
If you delete those then you should be back to a vanilla report when you open the report desinger again (you might need to close and re-open the workflow to see the changes)