device_decode is now no longer used, so we may as well remove it.
Fixes gcc 6 warning:

drivers/acpi/acpi_video.c:221:19: warning: ‘device_decode’ defined
 but not used [-Wunused-const-variable]
 static const char device_decode[][30] = {
                   ^~~~~~~~~~~~~
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>