Remove old flags

[pvinis]

We have some comments like old flag or remove once few or no users are on version X.Y.Z in Echo.json5.

We should take a look at https://artsy.looker.com/explore/frontend_analytics/eigen_tracks?qid=SZXeeacYh6Hq2CnE9ooGDl&origin_space=712&toggle=fil,vis every few months, and if few or no users are on that version, we could then remove the flag (if it says remove once..) or remove the old flag comment and treat the flag as a regular echo flag from then on.

Some context, #87, #89.