rgw: fix segfault in UserAsyncRefreshHandler::init_fetch

rgw: fix segfault in UserAsyncRefreshHandler::init_fetch

Fixes a segfault that was occuring in error handling code of UserAsyncRefreshHandler::init_fetch.
When ruser->read_stats_async returned an error code, the instance of UserAsyncRefreshHandler had
already been deallocated in RGWSI_User_RADOS::read_stats_async and a segmentation fault occurs
when attempting to print a member variable in error logs. This commit removes the extra ref count
drop since the ref is properly dropped upstream in RGWQuotaCache::async_refresh error handling
logic.

Fixes: https://tracker.ceph.com/issues/54112
Signed-off-by: Cory Snyder <csnyder@iland.com>
(cherry picked from commit 71ef3af870e5789e71480682f11a883ff3a673e7)