staging: comedi: drop serial2002 driver

There's not been any work on this driver since it was originally merged,
and it really needs to be rewritten to use the serdev layer instead if
people really need/want it.

Reported-by: Christoph Hellwig <hch@lst.de>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>