s390-pcibus: No need to set user_creatable=false explicitly

TYPE_S390_PCI_HOST_BRIDGE is a subclass of TYPE_PCI_HOST_BRIDGE,
which is a subclass of TYPE_SYS_BUS_DEVICE. TYPE_SYS_BUS_DEVICE
already sets user_creatable=false, so we don't require an
explicit user_creatable=false assignment in
s390_pcihost_class_init().

Cc: Alexander Graf <agraf@suse.de>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Frank Blaschka <frank.blaschka@de.ibm.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Pierre Morel <pmorel@linux.vnet.ibm.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-22-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>