Bug 20619: Remove last occurrences of long gone syspref (opacsmallimage)

Bug 20619: Remove last occurrences of long gone syspref (opacsmallimage)

Code was commented anyway, so it was useless

Test plan:
1. Read the diff
2. Confirm that it only removes comments
3. Confirm that there are no remaining references to this syspref in the
   code (only remaining references are in updatedatabase.pl and release
   notes)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>