Yes, we can remove this code.  This code was relevant when we were on the main thread and were using FileReader.  We transitioned to FileReaderSync on the worker because there is no FileReader and I/we hadn't realized we could use XHR.  And then I/we forgot to remove the dead code.  (I think that was me, but it might have been Vivien.  If it was Vivien, it was definitely my review bad.)