search: delete !hier! feature flag/code path

Created by: ijt

Our plan is to make a fresh start on adding hierarchical search. The implementation being deleted in this pull request was partial, and the people taking ownership of the feature (@ijt and @ijsnow) are not so familiar with how it was done. The amount of work involved in understanding it and completing it would not necessarily be less than it would be to start fresh. Also, a fresh start should allow us to take a more incremental approach that could lower the risk of introducing new bugs into search.

This is part of #2740.