Remove backward compatibility for importing `process_webhook()`

[jeremystretch]

Proposed Changes

extras.webhooks_worker.process_webhook() was renamed and moved to `extras.webhooks.send_webhook() in v3.7 under #14395, with backward compatibility being retained for importing the function from its previous location. This change removes that backward compatibility.

Justification

Removes obsolete code

Impact

Any plugins which utilize this function imported from its previous location & name will need to be updated to reference the new path. (This function is not supported for direct use by plugins, but we provided backward compatibility across v3.7 releases just in case.)