Revert "Add upvotes to merge requests documents"

What does this MR do and why?

Related to #263365

This is reverting changes from !66671 (merged) which led to an incident (more detail at): gitlab-com/gl-infra/production#5568 (closed)

We will re-evaluate whether these changes can be introduced safely (with modification) to reduce database performance for the affected queries while guaranteeing that we can index 16,000 documents in under 1 minute to Elasticsearch.

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