rgw: Drop Unused function in rgw_usage.cc

rgw: Drop Unused function in rgw_usage.cc

fix compile warnning in any PR (Ex: https://jenkins.ceph.com/job/ceph-pull-requests/29193/console):
/home/jenkins-build/build/workspace/ceph-pull-requests/src/rgw/rgw_usage.cc:33:13: warning: ‘void dump_usage_bucket_info(ceph::Formatter*, std::__cxx11::string&, cls_user_bucket_entry&)’ defined but not used [-Wunused-function]
 static void dump_usage_bucket_info(Formatter *formatter, string& name, cls_user_bucket_entry& entry)

Signed-off-by: Wei Qiaomiao <wei.qiaomiao@zte.com.cn>