config: remove GITHUB_WEBHOOK_RECEIVER_ID

[slint]

This config var seems only to be used for selecting the OAuth remote application config, 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.