MXS-1653: Remove false error message

Removed false error message about failed session commands. An error in
response to a session command is a perfectly valid result.

Also added the explicit commands that the master and slave return to the
warning that is logged when the results differ.

server/modules/routing/readwritesplit/readwritesplit.cc

@@ -1197,8 +1197,6 @@ static void clientReply(MXS_ROUTER *instance,
      */
     if (backend->session_command_count())
     {
-        check_session_command_reply(writebuf, backend);
-
         /** This discards all responses that have already been sent to the client */
         bool rconn = false;
         process_sescmd_response(rses, backend, &writebuf, &rconn);