Bug 24564: Do not modify the tab of existing subfield in ifla/update.pl

Bug 24564: Do not modify the tab of existing subfield in ifla/update.pl

Test plan:
1. Find a biblio subfield in
   misc/migration_tools/ifla/data/biblio/default.yml that you have in
   your default biblio MARC framework (or create one).
2. Change the tab of this subfield so that it's different from what's in
   the .yml file
3. Run misc/migration_tools/ifla/update.pl --force
4. Verify that the tab of this subfield has not been changed.
5. Do the same for authorities (files are in
   misc/migration_tools/ifla/data/auth/)

Signed-off-by: sonia <sonia.bouis@univ-lyon3.fr>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>