mruby-compiler: remove traditional node generation from new_back_ref

Remove if (!p->var_nodes_enabled) branch from new_back_ref function
to use variable-sized nodes exclusively for regex backreferences.

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

mrbgems/mruby-compiler/core/parse.y

@@ -2462,10 +2462,6 @@ new_dregx(parser_state *p, node *a, node *b)
 static node*
 new_back_ref(parser_state *p, int n)
 {
-  if (!p->var_nodes_enabled) {
-    return cons_head((node*)NODE_BACK_REF, int_to_node(n));
-  }
-
   size_t total_size = sizeof(struct mrb_ast_back_ref_node);
   enum mrb_ast_size_class class = size_to_class(total_size);
   struct mrb_ast_back_ref_node *backref_node = (struct mrb_ast_back_ref_node*)