After SERVER-63577, we will know definitively when a unique index contains only new-format keys.

This means we can skip this code that inserts and deletes a key every time we check for a duplicate.

This will significantly speed up inserts in two ways:
1. We don't have to perform this extra insert and delete
2. We can avoid performance issues with performing search_near when a lot of keys are not visible. See WT-8150.