Refactor the way car count is accessed

What does this MR do and why?

We are deprecating merge_train_count and merge_train_index on the merge request type, given we now have access to merge trains and cars directly in the API. This MR refactors our code to use the new data source.

Part one can be found here: !164241 (merged)

Part two: You are here 😸

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Verify data still displays correct for merge trains count/index
2. Grep for the replaced data to ensure it was refactored correctly

Related to #475269 (closed)