Bug 28893: Remove unused opac/rss directory

Bug 28893: Remove unused opac/rss directory

RSS feeds are now generated using opac/opac-search.pl (format=rss).

But prior to
  commit 09df0de35fcb5b1645490bd3ec9d9a77ec7923a1
  Removing obsolete RSS <link> tags and the scripts
it was using opac/opac-rss.pl, which used opac/rss

The directory should have been removed by this commit.

Test plan:
Confirm that this directory is useless and can be removed.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>