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