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