delete endpoint for pushers

The description of the `kind` parameter from the current version.
> The kind of pusher to configure. "http" makes a pusher that sends HTTP pokes. null deletes the pusher.

I would suggest creating a new endpoint like `DELETE /pusher` to delete a pusher and be more explicit about the intend instead of hiding it inside an arbitrary parameter.

Start by reading the current specification text for the pusher `kind` parameter and its deletion behavior. Assess whether a separate `DELETE /pusher` endpoint should replace or supplement that behavior, then update the relevant specification text and verify the endpoint and parameter semantics are consistent.

Written by the indexing model from the issue text.