[media] uvcvideo: Remove unneeded device disconnected flag

The UVC_DEV_DISCONNECTED flag is set when the USB device is
disconnected, and checked in the open() handler to refuse opening a
device that has been disconnected.

This is inherently racy, but the race condition is taken care of in the
V4L2 core, which also fails open() calls when the V4L2 device node has
been unregistered. As the uvcvideo USB disconnect handler unregisters
all its video device nodes, the UVC_DEV_DISCONNECTED flag doesn't serve
any purpose anymore and can be removed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>