mruby-compiler: remove traditional node generation from new_dxstr

Remove if (!p->var_nodes_enabled) branch from new_dxstr function
to use variable-sized nodes exclusively for dynamic execution strings.

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

mrbgems/mruby-compiler/core/parse.y

@@ -2404,10 +2404,6 @@ new_xstr(parser_state *p, const char *s, int len)
 static node*
 new_dxstr(parser_state *p, node *a)
 {
-  if (!p->var_nodes_enabled) {
-    return cons_head((node*)NODE_DXSTR, a);
-  }
-
   size_t total_size = sizeof(struct mrb_ast_dxstr_node);
   enum mrb_ast_size_class class = size_to_class(total_size);
   struct mrb_ast_dxstr_node *n = (struct mrb_ast_dxstr_node*)