Bug 16717: Remove hardcoded category from Holds.t

Bug 16717: Remove hardcoded category from Holds.t

There is actually no need to use the hardcoded patron category S in
this test.

Test plan:
Run t/db_dependent/Holds.t.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Test passes OK
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit a4690353d9a20a7c991787a1ffe3040eb3834a5a)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>