Go-github: Remove graduated GitHub v3 API preview headers

[gmlewis]

GitHub Developer v3 API announcement:
https://developer.github.com/changes/2020-08-20-graduate-machine-man-and-sailor-v-previews/

These custom preview headers have now graduated and need to be remove from this repo:

• application/vnd.github.machine-man-preview+json
• application/vnd.github.sailor-v-preview+json

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!

[vikkyomkar]

I would like to contribute to this

[guacamole]

hey @gmlewis, can i start working on this one?

[gmlewis]

Thanks, you two. It looks like @vikkyomkar was first, so it is yours.