media: usbvision: remove deprecated driver

To quote the TODO of this driver:

--------------------------------------------------------------
The driver is deprecated and scheduled for removal by the end
of 2020.

In order to prevent removal the following actions would have to
be taken:

- clean up the code
- convert to the vb2 framework
- fix the disconnect and free-on-last-user handling (i.e., add
  a release callback for struct v4l2_device and rework the code
  to use that correctly).
--------------------------------------------------------------

Nobody picked this up, so it's time to retire this driver.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>