pnfsmanager: Clean up cleaner by making use of new CellStub features

A workaround for CellStub limitations is no longer necessary.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6765/