remove partial implementation of nested search

Created by: ijt

It was confusing to have redundant code paths for search.

Most likely we'll want to refer back to the code being deleted in this change when we start working on nested search again, and also if we decide to separate the business logic from graphqlbackend.

Test plan: I manually ran some searches with index:yes and index:no.

CC @keegancsmith, @attfarhan, @kzh