Migrate Project workboard background color to modular transactions

Summary: Removes now-unused method as well. Fixes T12673.

Test Plan: UI fiddling.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T12673

Differential Revision: https://secure.phabricator.com/D18014

src/applications/project/controller/PhabricatorProjectBoardBackgroundController.php

@@ -37,7 +37,8 @@ final class PhabricatorProjectBoardBackgroundController
       $xactions = array();
 
       $xactions[] = id(new PhabricatorProjectTransaction())
-        ->setTransactionType(PhabricatorProjectTransaction::TYPE_BACKGROUND)
+        ->setTransactionType(
+            PhabricatorProjectWorkboardBackgroundTransaction::TRANSACTIONTYPE)
         ->setNewValue($background_key);
 
       id(new PhabricatorProjectTransactionEditor())