R1:f43d08c2bbe6

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