Bug 19097: Remove wantarray from GetMarcSubfieldStructureFromKohaField

Bug 19097: Remove wantarray from GetMarcSubfieldStructureFromKohaField

Replacing wantarray by always returning all mappings.
In a few cases only we expect multiple ones.
Changing two calls to pick the first hit, and add comment about
the implicit assumption being made (as before, no behavior change).

Test plan:
Look at results of git grep GetMarcSubfieldStructureFromKohaField
Run t/db_dependent/Biblio.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>