cls: remove unused variable

to silence following warning:

```
/home/kefu/dev/ceph/src/cls/rgw/cls_rgw_types.cc: In static member function ‘static std::__cxx11::list<rgw_bucket_dir> rgw_bucket_dir::generate_test_instances()’:
/home/kefu/dev/ceph/src/cls/rgw/cls_rgw_types.cc:736:11: warning: variable ‘i’ set but not used [-Wunused-but-set-variable=]
  736 |   uint8_t i = 0;
      |           ^
```

Signed-off-by: Kefu Chai <k.chai@proxmox.com>