Prune 15 obsolete ports

FreeBSD Ports maintainers have already deleted the following:
  editors/libreoffice-ca_XV
  editors/libreoffice-ku
  editors/libreoffice-sh
  german/bsdforen-firefox-searchplugin
  german/bsdgroup-firefox-searchplugin
  graphics/GraphicsMagick13
  lang/clojure-contrib
  lang/gnat-aux
  lang/ocaml-doc
  lang/ocaml-examples
  lang/python31
  multimedia/shell-fm
  textproc/m17n-contrib
  www/mediawiki120
  www/xpi-deepestsender