Remove StoppableOutputStream class used by VCFRecordWriter

[cmnbroad]

samtools/htsjdk#904 has a change that should eliminate the need for the writeHeader workaround currently employed by VCFRecordWriter. Once the htsjdk branch is merged, it will be possible to call setHeader on the writer to establish the header without actually writing it to the stream, which in turn should eliminate the need for StoppableOutputStream currently being used.