Remove unused element count field on worker versions

Found while working on #1116 (closed).

The element_count field was removed in https://gitlab.com/arkindex/backend/-/commit/50e7a00eb2d1b8c9b9bc1f9e9f57aa22f5171d5c?merge_request_iid=1442 when we switched to the corpus worker version cache (!1442 (merged)). It is still present in serializers, but is always set to null, and is not used anywhere according to code.vpn.

This is done in a separate MR because this affects every worker version endpoint and not just ListCorpusWorkerVersions.