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>