firmware: Remove newly-added slicoss and sxg firmware images

These are available elsewhere (for example in the linux-firmware.git
repository); they have no business being added to the kernel source
tree.

We are only putting stuff in the firmware/ directory of the kernel
source when it's extracted from long-standing drivers which used to
include it directly.

We didn't intend to open the floodgates to including megabytes of new
firmware which was previously being distributed separately.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>