block: remove skd driver

The STEC S1220 PCIe SSD cards are EOL since 2014 and not supported by
the vendor anymore. As the skd driver for this SSD is starting to cause
problems with improvements to the block layer, stop supporting it in
newer kernel versions.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>