rpc: deprecate Client.ShhSubscribe (#22239)

It never worked, whisper uses polling.

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/client.go

@@ -414,6 +414,7 @@ func (c *Client) EthSubscribe(ctx context.Context, channel interface{}, args ...
 }
 
 // ShhSubscribe registers a subscripion under the "shh" namespace.
+// Deprecated: use Subscribe(ctx, "shh", ...).
 func (c *Client) ShhSubscribe(ctx context.Context, channel interface{}, args ...interface{}) (*ClientSubscription, error) {
 	return c.Subscribe(ctx, "shh", channel, args...)
 }