skel: httpd.properties mark plots subdir as obsolete

module: skel

The httpd.static-content.plots.subdir has not been used since 2.3.

Target: master
Committed: master@d2fe542f0a4bd9218b7008ffe5890e6ba7818fde
Acked-by: Tigran
Patch: http://rb.dcache.org/r/5557
Require-notes: no
Require-book: no
Request: 2.6
Ticket: http://rt.dcache.org/Ticket/Display.html?id=7815