{mb,soc/intel/skylake}: remove unused InternalGfx

The InternalGfx option in devicetree.cb is not used to enable iGPU.
The patch removes this option from chip.h and mb/*/devicetree.cb
files for all boards with skl/kbl processor.

Change-Id: I41ecca3fdfb1d4b20ee634a13263ff481dcf440e
Signed-off-by: Maxim Polyakov <max.senia.poliak@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32171
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/intel/skylake/chip.h

@@ -318,7 +318,6 @@ struct soc_intel_skylake_config {
 		Display_Auto,
 		Display_Switchable,
 	} PrimaryDisplay;
-	u8 InternalGfx;
 	u8 ApertureSize;
 	u8 SkipExtGfxScan;
 	u8 ScanExtGfxForLegacyOpRom;