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>
(cherry picked from commit 2f522d8ad4d0ce4a05250063bcc1dcba9fabd7ee)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 1440eb532d4bd2abdc76b4955cabb492283737a7)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
(cherry picked from commit 6a89c2dd82f94ac5795c0e9b7abd6ce77733c902)
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>