bake: fix variadic_params inconsistency for user functions

There was inconsistency between variables used for function
definitions in HCL and JSON format. Updated JSON to match HCL,
fixed documentation and removed the unused code from userfunc
pkg (based on HCL upstream) to avoid confusion.

Theoretically we could add some temporary backwards compatibility
for the JSON format but I think it is unlikely that someone uses
JSON format for this and also defined variadic parameters.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/userfunc/README.md

@@ -13,7 +13,7 @@ function "add" {
 
 function "list" {
   params         = []
-  variadic_param = items
+  variadic_params = items
   result         = items
 }
 ```