Also move Benchmarker.php to maintenance/includes/ because
that file is not itself a benchmark, and that base class is
actually covered by tests and should remain included.
Change-Id: I4acd88242dde56a884d319dfc141a3511a8221a3
Also remove a few redundant file-level descriptions in favour
of their class entity describing the same already.
Change-Id: I1a43fc402b5bd106931062a399952ba1e48beb48