Remove notes count from network graph

What does this MR do and why?

Removes the notes count from network graph. This is an extremely minor feature (it just shows a count of comments on a commit, which most people don't even realise is possible) with a high performance impact. Removing this allowed the gitlab-org/gitlab network graph to finally load again, rather than timing out.

Changelog: performance

Related #382720 (closed)

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