rgw: drop unused assignment

rgw/rgw_rados.cc: In member function 'virtual int RGWPutObjProcessor_Atomic::handle_data(ceph::bufferlist&, off_t, void**)':
rgw/rgw_rados.cc:648:5: warning: parameter 'ofs' set but not used [-Wunused-but-set-parameter]

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>