docs/qdev-device-use: Clean up the sentences related to -usbdevice

Most of the -usbdevice paramaters have been removed already. Update
the doc accordingly.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20200710065520.24784-1-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>