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>

WHENCE

@@ -2834,9 +2834,6 @@ Licence: Redistributable. See LICENCE.cavium for details
 
 Driver: i915 -- Intel Integrated Graphics driver
 
-File: i915/skl_dmc_ver1_04.bin
-File: i915/skl_dmc_ver1_16.bin
-File: i915/skl_dmc_ver1_18.bin
 File: i915/skl_dmc_ver1_19.bin
 File: i915/skl_dmc_ver1_20.bin
 File: i915/skl_dmc_ver1_21.bin