Remove unused message handlers 

After searching in the code base for dead code I realized that we handle two messages that are never sent. The first is negotiate_pubkey and its response is response_pubkey. These handlers can be removed as well as the code that they were using in transport.py.
Is this [unrelated](https://github.com/OpenBazaar/OpenBazaar/blob/develop/html/controllers/market.js#L66)?