Remove remove_deployments_api_ref_sort

What does this MR do and why?

Related to [Feature flag] Rollout of `remove_deployments_a... (#416305 - closed)

Remove feature flag, remove ref from constant

In the original MR that added this feature flag (!124229 (merged)) it was simpler to handle the logic as the one liner rather than conditionally defining the constant to include or not include 'ref'.

Now that the feature flag is being removed and the behavior has been rolled out to production, I think it is simpler to remove the one liner and remove 'ref' from ALLOWED_SORT_VALUES.

The logic in the affected method build_sort_params already does the desired behavior if we remove 'ref' from the method.

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