Remove recently added repository_pathchange key

Summary:
Ref T10560. Reverts D15460. See that task for discussion: we dug up some more information to explain the behavior, and this key was just sort of sidestepping an analyze/cardinality estimate issue on the index.

With proper cardinality estimates it shouldn't be used, so just nuke it.

Test Plan: Ran `bin/storage adjust`, saw key drop.

Reviewers: eadler, chad

Reviewed By: chad

Maniphest Tasks: T10560

Differential Revision: https://secure.phabricator.com/D15486

src/applications/repository/storage/PhabricatorRepositorySchemaSpec.php

@@ -155,9 +155,6 @@ final class PhabricatorRepositorySchemaSpec
         'repositoryID' => array(
           'columns' => array('repositoryID', 'pathID', 'commitSequence'),
         ),
-        'key_history' => array(
-          'columns' => array('commitID', 'isDirect', 'changeType'),
-        ),
       ));
 
     $this->buildRawSchema(