openonuAdapterGo - delete requests for objects of an already deleted ONU should be acknowledged positively

While reviewing HW test logs, we noticed that InterAdapterMessages for a ONU device to delete GEM ports and TCONTs coming from the OLT adapter can arrive at the ONU adapter after the RPC from core to delete this device has already been processed. The corresponding InterAdapterMessages are currently acknowledged negatively by the ONU adapter in this case, which triggers unnecessary error handling in the calling OLT adapter. To avoid this, delete requests for objects of an already deleted ONU should be acknowledged positively.