tty/serial: remove the sn_console driver

The SGI SN2 support is about to be removed.  Remove this driver that
depends on the SN2 support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lkml.kernel.org/r/20190813072514.23299-7-hch@lst.de
Signed-off-by: Tony Luck <tony.luck@intel.com>