I think you can just try to run the setup for SQL Server again and it should give you an option to select the parts you want to keep. Reporting Services will be selected by default. Un check it and continue. That should uninstall Reporting Services.

If you just dont care about the files being present on the server, just disable the Windows service for ReportingServices and delete the virtual directories from IIS. This should make it as good as removed from that machine.