librados: drop unused AioCompletion in pool_io_callback.

librados: drop unused AioCompletion in pool_io_callback.

This change is to deal with:

```
[1048/1786] Building CXX object src/test/librados/CMakeFiles/ceph_test_rados_api_aio_pp.dir/aio_cxx.cc.o
../src/test/librados/aio_cxx.cc: In function ‘void pool_io_callback(librados::completion_t, void*)’:
../src/test/librados/aio_cxx.cc:2247:18: warning: unused variable ‘c’ [-Wunused-variable]
 2247 |   AioCompletion *c = (AioCompletion*)cb;
      |
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>