mainboard/facebook/fbg1701: Remove unused includes

Files onboard.h and mainboard.h are not used for building.
Remove these files as include.

BUG=N/A
TEST=Build and boot embedded Linux 4.20 on Facebook FBG-1701

Change-Id: Ifeb0047357e641cbe1affbbaf5402213802c774c
Signed-off-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33574
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/mainboard/facebook/fbg1701/mainboard.c

@@ -17,8 +17,6 @@
  */
 
 #include <device/device.h>
-#include "onboard.h"
-#include "mainboard.h"
 
 /*
  * Declare the resources we are using