Remove PhabricatorPHID::fromObjectName

Summary: Ref T2715. This only ever supported like 10% of object types; get rid of it in favor of the new infra.

Test Plan:

• Ran bin/search index D12; bin/search index <some valid phid>, bin/search index derp.
• Turned off Search jump, searched for D12.
• Used phid.lookup.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715

Differential Revision: https://secure.phabricator.com/D6519