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