refactor(api): return distinct publishers

[koeaw]

Return distinct() expression_publishers to fix
errors resulting from duplicate relations set
between Expressions and Organisations + remove
current workaround of slicing returned publishers.

[koeaw]

@sennierer As said on MM, I believe I've identified a(n accidental) duplication of the publisher relation between an Expression and Organisation as the problem which caused the error with too many publishers being returned.

This would filter out the same weirdness should it happen again – instead of slicing the publishers. Given we don't think there is/should/can be more than one publisher per Expression.

[koeaw]

The offending Expression entity is 26373, which is related to/caused errors for Works 19724 and 26613 when those were queried via the API.

ETA: One Work appears to be part of the other, based on their titles/descriptions. Which, IIRC, should result in "is realised in" relations to two distinct Expressions (one for each) and a link between the two Expressions ("expression is part of expression"/"expression has part expression").