[ncp] remove dead code in SPINEL_PROP_STREAM_NET set handler (#4475)

src/ncp/ncp_base_mtd.cpp

@@ -1959,18 +1959,8 @@ template <> otError NcpBase::HandlePropertySet<SPINEL_PROP_STREAM_NET>(void)
 
     error = otIp6Send(mInstance, message);
 
-    // `otIp6Send()` takes ownership of `message` (in both success or
-    // failure cases). `message` is set to NULL so it is not freed at
-    // exit.
-    message = NULL;
-
 exit:
 
-    if (message != NULL)
-    {
-        otMessageFree(message);
-    }
-
     if (error == OT_ERROR_NONE)
     {
         mInboundSecureIpFrameCounter++;