hw/usb/bus: Remove bad object_unparent() from usb_try_create_simple()

Valgrind detects an invalid read operation when hot-plugging of an USB device fails:

$ valgrind x86_64-softmmu/qemu-system-x86_64 -device usb-ehci -nographic -S
==30598== Memcheck, a memory error detector
==30598== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==30598== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==30598== Command: x86_64-softmmu/qemu-system-x86_64 -device usb-ehci -nographic -S
==30598==
QEMU 2.10.50 monitor - type 'help' for more information
(qemu) device_add usb-tablet
(qemu) device_add usb-tablet
(qemu) device_add usb-tablet
(qemu) device_add usb-tablet
(qemu) device_add usb-tablet
(qemu) device_add usb-tablet
==30598== Invalid read of size 8
==30598==    at 0x60EF50: object_unparent (object.c:445)
==30598==    by 0x580F0D: usb_try_create_simple (bus.c:346)
==30598==    by 0x581BEB: usb_claim_port (bus.c:451)
==30598==    by 0x582310: usb_qdev_realize (bus.c:257)
==30598==    by 0x4CB399: device_set_realized (qdev.c:914)
==30598==    by 0x60E26D: property_set_bool (object.c:1886)
==30598==    by 0x61235E: object_property_set_qobject (qom-qobject.c:27)
==30598==    by 0x61000F: object_property_set_bool (object.c:1162)
==30598==    by 0x4567C3: qdev_device_add (qdev-monitor.c:630)
==30598==    by 0x456D52: qmp_device_add (qdev-monitor.c:807)
==30598==    by 0x470A99: hmp_device_add (hmp.c:1933)
==30598==    by 0x3679C3: handle_hmp_command (monitor.c:3123)

The object_unparent() here is not necessary anymore since commit 69382d8b3e8600b3 ("qdev: Fix object reference leak in case device.realize() fails"), so let's remove it now.

---
 hw/usb/bus.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

index d910f84..e56dc33 100644
@@ -341,9 +341,7 @@ static USBDevice *usb_try_create_simple(USBBus *bus, const char *name,
     object_property_set_bool(OBJECT(dev), true, "realized", &err);
     if (err) {
         error_propagate(errp, err);
-        error_prepend(errp, "Failed to initialize USB device '%s': ",
-                      name);
-        object_unparent(OBJECT(dev));
+        error_prepend(errp, "Failed to initialize USB device '%s': ", name);
         return NULL;
     }
     return dev;

1.8.3.1