[Later] Drop legacyCommentID column from DifferentialTransactionComment

Summary: Ref T2222. No callsites. Holding until we're more clearly stable.

Test Plan: `grep`

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8241

src/applications/differential/storage/DifferentialTransactionComment.php

@@ -11,7 +11,6 @@ final class DifferentialTransactionComment
   protected $fixedState;
   protected $hasReplies = 0;
   protected $replyToCommentPHID;
-  protected $legacyCommentID;
 
   public function getApplicationTransactionObject() {
     return new DifferentialTransaction();