Remove feature flags related to FF merge trains

This removes the flags `merge_trains_create_ref_service` and
`fast_forward_merge_trains_support`, which controlled the rollout of
fast-forward and semi-linear merge trains. Both flags have been enabled
everywhere since 16.5.

The compatibility code paths should be removed at some point, but we
leave them in for now to ensure a smooth upgrade from any 16.x to 17.x.

Changelog: other