usb-serial: Remove double call to qemu_chr_add_handlers( NULL )

usb-serial has a qdev chardev property, and hw/qdev-properties-system.c
already contains:

static void release_chr(Object *obj, const char *name, void *opaque)
{
    DeviceState *dev = DEVICE(obj);
    Property *prop = opaque;
    CharDriverState **ptr = qdev_get_prop_ptr(dev, prop);
    CharDriverState *chr = *ptr;

    if (chr) {
        qemu_chr_add_handlers(chr, NULL, NULL, NULL, NULL);
        qemu_chr_fe_release(chr);
    }
}

So doing the qemu_chr_add_handlers(s->cs, NULL, NULL, NULL, NULL); from
the usb handle_destroy function too will lead to it being done twice.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-serial.c

@@ -410,13 +410,6 @@ static void usb_serial_handle_data(USBDevice *dev, USBPacket *p)
     }
 }
 
-static void usb_serial_handle_destroy(USBDevice *dev)
-{
-    USBSerialState *s = (USBSerialState *)dev;
-
-    qemu_chr_add_handlers(s->cs, NULL, NULL, NULL, NULL);
-}
-
 static int usb_serial_can_read(void *opaque)
 {
     USBSerialState *s = opaque;
@@ -595,7 +588,6 @@ static void usb_serial_class_initfn(ObjectClass *klass, void *data)
     uc->handle_reset   = usb_serial_handle_reset;
     uc->handle_control = usb_serial_handle_control;
     uc->handle_data    = usb_serial_handle_data;
-    uc->handle_destroy = usb_serial_handle_destroy;
     dc->vmsd = &vmstate_usb_serial;
     dc->props = serial_properties;
 }
@@ -623,7 +615,6 @@ static void usb_braille_class_initfn(ObjectClass *klass, void *data)
     uc->handle_reset   = usb_serial_handle_reset;
     uc->handle_control = usb_serial_handle_control;
     uc->handle_data    = usb_serial_handle_data;
-    uc->handle_destroy = usb_serial_handle_destroy;
     dc->vmsd = &vmstate_usb_serial;
     dc->props = braille_properties;
 }