Remove unused go/helper package
This removes unused library code. We know it's unused because it wasn't even compiling anymore, and we did not notice anywhere in the GitLab build -- see gitaly#1427 (closed).
This code has been superseded by https://gitlab.com/gitlab-org/gitaly/tree/master/streamio
Closes gitaly#1427 (closed)
Edited by Jacob Vosmaer