linux-firmware: Clean up i915 by removing old skl dmc firmware.

linux-firmware: Clean up i915 by removing old skl dmc firmware.

The code points to the major version that is a link to the most
recent one, so we can remove the old firmware blobs.

For tests and validation purposes we will let at least 3 latest
minor available, but we need to let linux-firmware repo as
clean as possible, so let's remove old and unused ones.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>