Remove merge_when_checks_pass_merge_train feature flag

What does this MR do and why?

This feature flag has been turned on by default since 3 months (#470667 (comment 2080590924))

The first step is to remove this feature flag and stop allowing the Add To Merge Train When Pipelines Succeeds service to be available. In this case, we can continue to be backward compatible with auto merges currently set to this strategy, without allowing more Merge Requests to use it.

In follow up MR's, we will remove the strategy as well.