Refactor ArbitrageAgent `answer_binary_market`

[gabrielfior]

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

To do once https://github.com/gnosis/prediction-market-agent/pull/511 is merged

[kongzii]

If https://github.com/gnosis/prediction-market-agent-tooling/pull/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