staging: remove intel_sst driver

Intel has asked that this driver now be removed from the tree, and I am
happy to oblige.

Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>