Remove custom media type for global node IDs

GitHub Developer API announcement:
https://developer.github.com/changes/2018-05-30-end-jean-grey-preview/

The custom media type `application/vnd.github.jean-grey-preview+json` can be removed from `github.go` as it is no longer needed. Also please verify that `application/vnd.github.v3+json` is being sent in the header.