rgw/rgw_main: fix unnecessary variables defined

Signed-off-by: zhang.zezhu <zhang.zezhu@zte.com.cn>

src/rgw/rgw_main.cc

@@ -78,9 +78,8 @@ void signal_shutdown()
     int val = 0;
     int ret = write(signal_fd[0], (char *)&val, sizeof(val));
     if (ret < 0) {
-      int err = -errno;
       derr << "ERROR: " << __func__ << ": write() returned "
-           << cpp_strerror(-err) << dendl;
+           << cpp_strerror(errno) << dendl;
     }
   }
 }