Remove custom media type for Merge Methods API

[gmlewis]

GitHub Announcement:
https://developer.github.com/changes/2017-04-07-end-merge-methods-api-preview/

The application/vnd.github.polaris-preview custom media type can now be removed.

Regarding the note about squash vs merge_method, this has already been addressed in #446.

[RaviTezu]

Can we assign this to @ahmedhagii ?
I see #621 is work in progress.
@shurcooL @gmlewis

[gmlewis]

Invite sent.

[dmitshur]

That PR is 90% finished but he's not responding. I think we should just carry it ourselves.

[RaviTezu]

@shurcooL @gmlewis
I can take this up, if there are no concerns.

[dmitshur]

I think we should just finish the existing PR and merge that. I'll do it now.

[dmitshur]

This is resolved now. Thanks for the offer to help @RaviTezu! If you want to work on another issue, we still have many open, so feel free.