Kconfig: Remove obsolete Kconfig symbols from google/intel boards

- CACHE_ROM is no longer used in the coreboot code.  It was removed in
commit 4337020b (Remove CACHE_ROM.)
- CAR_MIGRATION is also no longer used in coreboot code - it was removed
in commit cbf5bdfe (CBMEM: Always select CAR_MIGRATION)
-  MARK_GRAPHICS_MEM_WRCOMB was removed in
commit 30fe6120 (MTRR: Mark all prefetchable resources as WRCOMB)

Change-Id: I8b33a08c256f6b022e57e9af60d0629d9a3ffac8
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: http://review.coreboot.org/12327
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/intel/skylake/Kconfig

@@ -14,8 +14,6 @@ config CPU_SPECIFIC_OPTIONS
 	select BACKUP_DEFAULT_SMM_REGION
 	select CACHE_MRC_SETTINGS
 	select CACHE_RELOCATED_RAMSTAGE_OUTSIDE_CBMEM if RELOCATABLE_RAMSTAGE
-	select CACHE_ROM
-	select CAR_MIGRATION
 	select COLLECT_TIMESTAMPS
 	select CPU_INTEL_FIRMWARE_INTERFACE_TABLE
 	select CPU_MICROCODE_IN_CBFS