Disable notes count on network graph

What does this MR do and why?

Describe in detail what your merge request does and why.

Experimentally disabling the notes count on the network graph endpoint. This query can take an extremely long time to return, and provides very little in terms of functionality. I've put this behind an experimental feature flag for the moment so we can see the impact in performance tests and on known slow projects on .com. I'd consider the endpoint not timing out a more useful feature than seeing a count of notes for a commit somewhere in the list (which seems to be so rarely used I can't even see any examples in our own projects).

Related #15263

Related #343018 (closed)

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