Remove MergeTrains::Car.total_count_in_train

What does this MR do and why?

Remove MergeTrains::Car.total_count_in_train

We now have MergeTrains::Train#car_count for this purpose, which is better organized than this class method

How to set up and validate locally

It's just dead code. You can check the test suite and globally search for any remaining method usage. It should all be gone.

The tests for this code were copied over pretty directly into the spec for MergeTrain#car_count which is now what we use.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #298541 (closed)