remove hex encoding of omci messages in olt adapter after fixing this in openolt agent

The olt adapter sends the omci messages as hex encoded strings because the agent expects 

the omci messages to be hex encoded string, As per the comment in openolt adapter device_handler.go(sendProxiedMessage), the agent code needs to be fixed to support the actual bytes instead of hex encoded string, and then changing the olt adapter to remove the hex encoding for omci messages, this story is created

1.  fix the openolt agent to receive omci messages as normal bytes instead of hex encoded string

2. remove hex encoding of omci messages in olt adapter.