Move back to using ohai from Rubygems
Chef devs have released a new version with our fixes - https://rubygems.org/gems/ohai/versions/14.14.0 . So we can go back to using Rubygems as SSOT.
This MR reverts most of the changes brought in by !3344 (merged). I have left the .custom_sources.yml
file changes there, so we can use it again if we find ourselves in a similar situation.