`ByteRangeStream` does not limit `CopyToAsync(...)` range as it should

[dougbu]

Fix is to remove the CopyToAsync(...) override in DelegatingStream. The underlying Stream's implementation of this method will do the Right Thing:tm: and use methods that are correctly overridden in DelegatingStream and its subclasses, including ByteRangeStream.

Issue originally tracked in https://devdiv.visualstudio.com/DevDiv/_workitems/edit/690342