Go-github: Remove custom preview headers from "graduated" APIs

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2019-12-13-graduated-previews-announced/

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started.

Thank you!

[vaibhavsingh97]

would love to work on this issue

[gmlewis]

Thank you, @vaibhavsingh97!
It's yours.