Bug 17152: Do not copy value when duplicating a subfield

Bug 17152: Do not copy value when duplicating a subfield

When cataloguing, if you want to duplicate a subfield that is not
empty, the new subfield is created with a copy of data in it.
This is not the case when you duplicate an whole field. The new one is
created with subfields but without data in it.

Test plan:
Add or edit a bibliographic record
Fill a subfield
Duplicate the subfield
=> Without this patch the value of the input will be copied
=> With this patch the new input will be emptied

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>