Remove migra/schemainspect

[ewjoachim]

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Because of https://github.com/djrobstep/schemainspect/issues/91, it's about to be more and more annoying to continue using migra.

https://github.com/stripe/pg-schema-diff does the same, as a go executable, so it's not ideal but it might be an alternative.

Otherwise, I wonder if using psql, export and diff the schema might be enough. The best dependency is no dependency :)