Remove unique constraints from flow_*_ref
The unique constraints are actually on all columns.
We don't really need this:
- it's not a proper way to ensure unique data
- code already inserts only uniques
- doesn't really matter if we were to have duplicates in there, that data is just a denormalization anyway (the real data can be extracted from flow_revision.rev_content)
But I've added an extra check just to make sure that duplicates,
which should never even make it in, won't show up.