Bug 20063: (bug 18811 follow-up) Allow hidden $9 subfields

Bug 20063: (bug 18811 follow-up) Allow hidden $9 subfields

Although we could argue that the user should make all $9 subfields
visible, this might be a good compromise. It selectively reverts the
change from the first patch by accepting $9 subfields without checking
the hidden flag in the framework.

Test plan:
[1] Make a subfield $9 visible for some authority field.
[2] Put a value in it somewhere in authorities.pl
[3] Make subfield $9 invisible in the auth framework.
[4] Open and save the adjusted authority record.
[5] Check the marcxml of this record.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>