Refactor ArbitrageAgent answer_binary_market

[gabrielfior]

Implement as @kongzii suggested (or come up with more creative implementation)

To do once #511 is merged

[kongzii]

If gnosis/prediction-market-agent-tooling#525 goes through in the review, then asnwer_and_build_trades function won't be possible anymore.

But I still think the goal here is doable, as a potential solution, I think we can do this:

Then ArbitrageAgent can implement custom build_trades (as it's doing already) that doesn't care about the answer. And ArbitrageAgent can implement answer_binary_market simply as return None