Completely remove the legacy hunk table

Summary: Depends on D19056. Fixes T8475. Ref T13054. Merges "ModernHunk" back into "Hunk".

Test Plan: Grepped for modernhunk. Reviewed revisions. Created a new revision. Used bin/differential migrate-hunk to migrate hunks between storage formats and back.

Maniphest Tasks: T13054, T8475

Differential Revision: https://secure.phabricator.com/D19057