plugin: Unwrap the rpc_command payload

We were nesting like the following:

```json
{"params": {
  "rpc_command": {
    "rpc_command": {
    }
  }
}
```

This is really excessive, so we unwrap once, and now have the following:

```json
{"params": {
  "rpc_command": {
  }
}
```

Still more wrapping than necessary (the method is repeated in the `params`
object), but it's getting closer.

Changelog-Deprecated: JSON-RPC: Removed double wrapping of `rpc_command` payload in `rpc_command` JSON field.

Suggested-by: @fiatjaf
Signed-off-by: Christian Decker <@cdecker>

tests/plugins/rpc_command.py

@@ -9,7 +9,7 @@ plugin = Plugin()
 
 @plugin.hook("rpc_command")
 def on_rpc_command(plugin, rpc_command, **kwargs):
-    request = rpc_command["rpc_command"]
+    request = rpc_command
     if request["method"] == "invoice":
         # Replace part of this command
         request["params"]["description"] = "A plugin modified this description"