ref(grouping): Remove hierarchical code from `find_existing_grouphash_new`

[lobsterkatie]

This PR is a follow-up to https://github.com/getsentry/sentry/pull/64858, which removed all hierarchical grouping code from _save_aggregate_new. This does the same for find_existing_grouphash_new. In both cases, we're able to do this because these functions will only be called for events not using the mobile config (and therefore not ever producing hierarchical hashes).

This is part of an effort to clean up and simplify these functions as much as possible before changing their logic, so that change can be as safe as possible.