Summary: Remove the `PhabricatorDefaultSearchEngineSelector` class. This is quite similar to D12053.
Test Plan: Went to `/view/PhabricatorSearchApplication/` and saw the storage engine configuration. Set `search.elastic.host` and saw the highlighted storage engine change.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D12670
