Enable routable_two_step_lookup permanently

What does this MR do?

This removes the feature flag routable_two_step_lookup and permanently enables the new behavior added by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32272.

Screenshots

From https://gitlab.com/gitlab-org/gitlab-ee/issues/30120#note_216773040:

The below graphs validate the effect of enabling the feature flag. We can see that the frequency of the heavier method where_full_path_in decreased significantly with this change.

Ratio of "bad" to "good" method calls:

Absolute numbers for method calls:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry