rpc: deprecate Client.ShhSubscribe

[gballet]

It never worked, whisper uses polling.

[fjl]

We can't remove this method because it has been around for quite a while and I don't want to break the API.
However, we can mark it deprecated. To do this, add a comment like

// ShhSubscribe registers a subscripion under the "shh" namespace.
// Deprecated: use Subscribe(ctx, "shh", ...) instead.

[gballet]

@fjl it's not going to break anything, this function never worked: whisper didn't use the subscription api.

[fjl]

This method is used in package shhclient: https://github.com/ethereum/whisper/blob/master/shhclient/client.go#L171

[gballet]

and yet, it never worked