Remove redundant error handling code from routers

The routers no longer need to track the number of errors each DCB
receives. This is now done by the protocol modules.

The type of the DCB no longer needs to be checked in the handleError
implementation as the function is only called when a backend DCB fails.

server/modules/routing/hintrouter/hintroutersession.cc

@@ -51,5 +51,6 @@ void HintRouterSession::handleError(GWBUF*             pMessage,
                                     mxs_error_action_t action,
                                     bool*              pSuccess)
 {
+    ss_dassert(pProblem->dcb_role == DCB_ROLE_BACKEND_HANDLER);
     MXS_ERROR("handleError not implemented yet.");
 }