mruby-compiler: update new_undef and new_negate to remove traditional node paths

- Modified new_undef to accept node *syms list instead of single mrb_sym
- Simplified gen_undef_var to directly pass symbol list
- Removed traditional node generation path from new_negate
- Both functions now use variable-sized nodes exclusively

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/parse.y

@@ -2635,10 +2635,6 @@ call_with_block(parser_state *p, node *a, node *b)
 static node*
 new_negate(parser_state *p, node *n)
 {
-  if (!p->var_nodes_enabled) {
-    return cons_head((node*)NODE_NEGATE, n);
-  }
-
   size_t total_size = sizeof(struct mrb_ast_negate_node);
   enum mrb_ast_size_class class = size_to_class(total_size);
   struct mrb_ast_negate_node *negate_node = (struct mrb_ast_negate_node*)