Remove Auth.newAuth as it has been discontinued

[sloshy]

As described here: https://docs.github.com/en/free-pro-team@latest/rest/overview/other-authentication-methods#via-username-and-password

Username/password auth has been discontinued, and this has inadvertently caused the build to start failing about a month ago. The API should be removed entirely, including any models or functions that this would have relied on.