Follow up: Remove `disable_merge_trains` flag

The following discussion from !46357 (merged) should be addressed:

• @marcel.amirault started a discussion: (+1 comment)

  @allison.browne It doesn't seem like this MR touches the disable_merge_trains feature flag, but at first glance it seems like this feature flag will no longer have any purpose? This was a "blocking" feature flag (my words), in that you turned the flag on to block/disable merge trains (which allowed merged results pipelines without merge trains, like in the gitlab project)...

  Then again, I have no idea how you'd pull it out... would that be a breaking change? Or can there be a migration that figures it out when you update, and sets the checkboxes accordingly? 🤔

  Ok, maybe this is something for a followup, but we'll probably need to update this section explaining how it'll work if you are using this feature flag when you update to 13.6...