Revert "Use setSVGData() to properly clear a costume that failed to load...

> Since the asset failed to load there is a good chance that it was never saved properly. This would account for the MD5 (generated by the editor) being in the project data but not having content for it on the server.

Except we *do* have content for it on the server. https://github.com/LLK/scratch-flash/issues/403#issuecomment-50527212