Bug 25440: Fixed list of 'show_rule' forming variables in the template

Bug 25440: Fixed list of 'show_rule' forming variables in the template

In smart-rules.tt we have `SET show_rule = ...` section which filled
with 'all used in the loop' variables. Because if historical reasons it
seems that there are some missing, few old, and even doubled ones.
This list is fixed now by:

  - variable names 'article_requests' and 'renewalsallowed' repeated
    so duplicates are removed;

  - 'hardduedatebefore' and 'hardduedateexact' not present in the whole
    site code anywhere anymore;
    IMPORTANT NOTE: these 'hardduedatebefore/hardduedateexact' also
                    exists as remnants in .po-translation files, a lot.

  - 'note', 'hardduedatecompare', 'renewalperiod', 'rentaldiscount'
    template variables were missing from this 'show_rule =' checking
    code so they are added.

Order of fields updated to match with above "SET field = ..." pack.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Bug 25440: (QA follow-up) Correction for typo

'engthunit -> lengthunit

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>