jsonrpc: use tal destructor to remove json commands when required.

This fixes a bug with a plugin duplicating an existing name
where we'd crash, too.

This doesn't work for builtins, which aren't tal objects, so
create a separate path for them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/jsonrpc.h

@@ -168,17 +168,11 @@ void jsonrpc_listen(struct jsonrpc *rpc, struct lightningd *ld);
  *
  * Returns true if the command was added correctly, false if adding
  * this would clobber a command name.
+ *
+ * Free @command to remove it.
  */
 bool jsonrpc_command_add(struct jsonrpc *rpc, struct json_command *command);
 
-/**
- * Remove a command/method from the JSON-RPC.
- *
- * Used to dynamically remove a `struct json_command` from the
- * JSON-RPC dispatch table by its name.
- */
-void jsonrpc_command_remove(struct jsonrpc *rpc, const char *method);
-
 /**
  * Begin a JSON-RPC notification with the specified topic.
  *