plugins: remove unused json_buffer / json_toks members from libplugin-pay.

They were never referenced, and saving the toks is questionable since their
lifetime is not guaranteed to live beyond this call (at least the buffer was
duplicated, but that also assumed it was at the start of the object).

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

plugins/libplugin-pay.h

@@ -148,9 +148,6 @@ struct payment {
 	struct plugin *plugin;
 	struct node_id *local_id;
 
-	const char *json_buffer;
-	const jsmntok_t *json_toks;
-
 	/* The current phase we are in. */
 	enum payment_step step;