My parser doesn't support default=, and I think it's safe to assume nobody is
running an instance whose DB schema version is one of those first 5 where
default= exists, so I'm remvoing it from the original 2016 model file. The
unset-default migrations remain, and I checked in `psql` that PostgreSQL
accepts the operation on columns that don't have a default value. If it turns
out to be a problem, I can replace those migrations with no-op ones.
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