fundchannel: Remove the now disabled deprecated satoshi param

While we removed the `satoshi` param in ElementsProject#3603 it appears that the
`fundchannel` plugin was still passing it to the `fundchannel_start`
call. This removes that shim and fixes up the help text. Notice that
technically the help text changes the param name, but since it was internally
always called `amount` this change doesn't break the API, the help was just
wrong.

plugins/fundchannel.c

@@ -274,8 +274,6 @@ static struct command_result *fundchannel_start(struct command *cmd,
 
 	json_add_string(req->js, "id", node_id_to_hexstr(tmpctx, fr->id));
 
-	if (deprecated_apis)
-		json_add_string(req->js, "satoshi", fr->funding_str);
 	json_add_string(req->js, "amount", fr->funding_str);
 
 	if (fr->feerate_str)
@@ -447,7 +445,7 @@ static void init(struct plugin *p,
 static const struct plugin_command commands[] = { {
 		"fundchannel",
 		"channels",
-		"Fund channel with {id} using {satoshi} (or 'all'), at optional {feerate}. "
+		"Fund channel with {id} using {amount} (or 'all'), at optional {feerate}. "
 		"Only use outputs that have {minconf} confirmations.",
 		"Initiaties a channel open with node 'id'. Must "
 		"be connected to the node and have enough funds available at the requested minimum confirmation "