rgw/swift: remove redundant moves of object name

rgw/swift: remove redundant moves of object name

resolves a compiler warning:

/home/cbodley/ceph/src/rgw/rgw_rest_swift.cc: In member function ‘RGWOp* RGWSwiftWebsiteHandler::get_ws_listing_op()’:
/home/cbodley/ceph/src/rgw/rgw_rest_swift.cc:2558:33: warning: redundant move in initialization [-Wredundant-move]
 2558 |   std::string prefix = std::move(s->object->get_name());
      |                        ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~
/home/cbodley/ceph/src/rgw/rgw_rest_swift.cc:2558:33: note: remove ‘std::move’ call

Signed-off-by: Casey Bodley <cbodley@redhat.com>