Remove default train status

What does this MR do and why?

There was a bug behind the feature flag :merge_trains_viz due to a default active status for trains. The bug resulted in merged cars not being fetched for a train while an active one was present.

When there is a running active train, we can not fetch by train status for merged cars within a train. This is because the train is no longer completed for the given target branch. This change allows us to fetch merged cars within a train omitting the status.

This is marked experimental and should not be a breaking change.

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. Enable merge trains and the FF merge_trains_viz
2. Have merged MRs via a train
3. Add an MR to the train and click "View merge details" in the widget on an MR.
4. Verify that while there is an active train the MR shows up on the active tab and the merged tab has all of the merged MRs for a train