Nit: Cleanup "BackendFailures" in controller.cpp

[strseb]

Time for some cleanup.

Description

This was the old hearbeat check

void MozillaVPN::heartbeatCompleted(bool success) {
  logger.debug() << "Server-side check done:" << success;

  if (!success) {
    m_private->m_controller.backendFailure();
    return;
  }

  if (m_state != StateBackendFailure) {
    return;
  }

  if (!modelsInitialized() || m_userState != UserAuthenticated) {
    setState(StateInitialize);
    return;
  }

  maybeStateMain();
}

So there we actually needed the controller to deativate and come back later.
Nowadays we only care during auth for heartbeat, we so vpn can't be activated anymore. So let's remove the controller code and rename all the screens to heartbeat failure, as otherwise a confused engineer might think this could have something to do with the daemon :)

[strseb]

There is actually a force_heartbeat_command aswell, so both did the same things 🫠

[mcleinman]

👍🏻