Bug 31214: Make subfield code non-editable in MARC framework editor

Bug 31214: Make subfield code non-editable in MARC framework editor

The subfield code field became editable in framework
editor due to a regression that happened during the
switch from a previous template framework koha used
to the new Template toolkit framework currently in use:
Bug 5917 : Swapping templates over (5884fb100071e973daf671a4ada6b32724376768)
The regression happened due to the template syntax
not being updated to the new one in this particular
case.

Note: The subfield code field doesn't align with the
      other fields anymore after this patch. However,
      this alignment issue is in every other Koha form
      in intranet and should be fixed in the global
      CSS rules.

1) Apply patch
2) Browse to
   /cgi-bin/koha/admin/marc_subfields_structure.pl?op=add_form&tagfield=995&frameworkcode=AR
3) Notice the subfield code is non-editable
4) Notice the subfield code is still editable in the New tab.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>