transactions: Enable majority-wins voting strategy by default

In order to get our feet wet with transactions, we initially enabled transactions with a primary-wins voting strategy. This means that as long as the primary casts a vote, the transaction is guaranteed to be committed. This of course doesn't buy us any consistency guarantees, but it did allow initial testing of the code without too much risk.

Given that primary-wins feature flag has been disabled in production for some time now without any outfall at all, let's change its default value to "disabled" to enable the majority-wins voting strategy by default.