refactor(ocpp20): remove redundant lastFirmwareStatus from per-station state
lastFirmwareStatus in the WeakMap was a duplicate of
stationInfo.firmwareStatus (now written by sendFirmwareStatusNotification).
TriggerMessage now uses hasFirmwareUpdateInProgress() to determine
whether to send Idle or the current intermediate state, which also
fixes a bug where failure end states (DownloadFailed, InstallationFailed,
etc.) were returned instead of Idle.