Revert "Fix hard linking issue causing overlay fs copy-ups"

This reverts commit 9ef0a9e8aa.

Master now has a better solution.

src/libstore/local-overlay-store.hh

@@ -186,12 +186,6 @@ private:
      */
     std::pair<bool, StorePathSet> verifyAllValidPaths(RepairFlag repair) override;
 
-    /**
-     * For lower-store paths, we used the lower store location. This avoids the
-     * wasteful "copying up" that would otherwise happen.
-     */
-    Path toRealPathForHardLink(const StorePath & storePath) override;
-
     /**
      * Deletion only effects the upper layer, so we ignore lower-layer referrers.
      */