R1:16a8ed72bdda

Modernize search engine selection

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