Replacing the "Delete reactions" endpoint

See https://developer.github.com/changes/2020-02-26-new-delete-reactions-endpoints/

Start with the linked GitHub API change notice and locate the existing delete-reactions endpoint in this PHP client. Compare the current client entry point with the replacement endpoints described there, then verify that the old endpoint is replaced and the relevant tests pass.

索引モデルが issue の本文から書いたものです。