Remove provision for missing fields on Hit

[dhruvkb]

Description

This PR removes the handling for Hit instances from the BaseModelSerializer because we no longer serialize hits from ES directly but rather always map them to model instances using the UUID identifier.

Testing Instructions

0. Check out the PR and run the dev server.
1. Check that API responds correctly to the requests.

The integration tests for the API should cover most scenarios, including comprehensive checking of example responses so a deep examination is not needed.

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[sarayourfriend]

Yay!

[sarayourfriend]

@dhruvkb should this PR also update the serializer to remove handling for ES hits now? Or is that a separate issue?

[dhruvkb]

I made an issue #4033 to find and remove all such occurrences. I didn't actually look for handling for Hits when I found this, only came across this during "Document media properties" and thought to make a small targeted PR.

[obulat]

LGTM