config: remove `GITHUB_WEBHOOK_RECEIVER_ID`

This config var seems only to be used [for selecting the OAuth remote application config](https://github.com/inveniosoftware/invenio-github/blob/01c27d6bca88cb1288136b96b17175b21da43d9d/invenio_github/api.py#L104), so naming-wise it's wrong, and besides this, I wouldn't expect actually that we would ever use a remote different than the `github` one (for a module called `invenio-github`).

IMHO we can remove the variable and hardcode the `github` string it where it used to be. If the module ends up becoming more generic (e.g. to support GitLab), we would anyways require a pretty heavy refactor.