Transitive closure should not include return and call links!

[aiida-bot]

Originally reported by: Giovanni Pizzi (Bitbucket: pizzi, GitHub: giovannipizzi)

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Change the triggers so that only those links are considered.

This should be relatively easy to do.
This could also be a good occasion for a cleanup of non-postgres code (we might still want to keep the triggers for the other backends somewhere, since it took us quite some time to write them).

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• Bitbucket: https://bitbucket.org/aiida_team/aiida_core/issue/298

[giovannipizzi]

Triggers don't exist anymore. #2220 also fixes the behaviour of the querybuilder when following with_ancestor