Draft: Remove the on-by-default ReferenceTransactions feature flag

The ReferenceTransactions flag has been on by default since 2074eaf5 (Revert "featureflag: Remove reference transaction feature flag", 2020-12-07), and the non-transaction code for the Ruby codepath got removed before that in d3ddbdba (hooks: Remove the Ruby reference-transaction hook feature flag, 2020-12-04).

It looks like we can remove it now, and since most of the if/else around keeping this flag was in tests I've worked on actively I was interested in removing it to speed them up, and make further changes on them easier.