When we used to allow cmd to be NULL, we had to hand the plugin
everywhere. We no longer do.
1. Various jsonrpc_ functions no longer need the plugin arg.
2. send_outreq no longer needs a plugin arg.
3. The init function takes a command, not a plugin.
4. Remove command_deprecated_in_nocmd_ok.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>