Drop partition indexer related code which is no longer used

As a first step.

Next step would be to rewrite the tests on content indexers since they are currently using the old run method... Which is no longer the way it's running in production (it's using the process_... whatever is the name which reads data from kafka).

Refs. #4733

mentioned in issue #4733

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approved this merge request

farewell, partition indexers!

merged