dvc: http resume doesn't really work

[Suor]

Having this on master branch.

If you look at RemoteHTTP._download_to() you will see that it always makes a full GET request, with no use of Range header. It simply skips previously downloaded part in ._write_request_content()

Pinging original author @pared.

[Suor]

We can fix this bug by dropping the functionality. I mean - it was broken from the start and nobody complained - maybe it is not really important for anyone.