FREE_RESPONSE is no longer a market type

[jg-you]

Free responses were changed to multiple choice with "addAnswersMode"

• References here: https://discord.com/channels/915138780216823849/1242910590083793036/1242926176624705666
• Notice what was previously coded as a Free response: https://api.manifold.markets/v0/slug/monthly-paper-search-1-which-ai-pap-57c34bfc81c7
• API reference: https://api.manifold.markets/v0/slug/monthly-paper-search-1-which-ai-pap-57c34bfc81c7

This breaks several tests.

I am writing a PR, and I'm happy to contribute the code, though I'm curious about your attitude towards backward compatibility.

My proposal is not backward compatible, it

• deletes the FreeResponseMarket type
• possibly re-orders the columns of the probabilities matrix (which did not come with an "index" anyways). This is because bets are linked by an answerId, and not outcome directly. One could make the change backward compatible by mapping outcomes to answer first, bit of overhead.

If that's all fine with you I'll put in the PR.