arch/x86/cache.h: Use ENV_CACHE_AS_RAM macro

Change-Id: Ic7b088a04165bb24b9ebcebc1580a96ce0fdfcc8
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37063
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/cpu/x86/car.c

@@ -65,11 +65,6 @@ void *car_get_var_ptr(void *var)
 	return &migrated_base[offset];
 }
 
-int car_active(void)
-{
-	return !car_migrated;
-}
-
 static void do_car_migrate_variables(void)
 {
 	void *migrated_base;