soc/intel/skylake: Drop dead `ScanExtGfxForLegacyOpRom`

This devicetree option is never set and never used. Drop it.

Change-Id: I9cd4733746849728b2b9f85793eace9191a97f49
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56207
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/soc/intel/skylake/chip.h

@@ -283,7 +283,6 @@ struct soc_intel_skylake_config {
 		Display_Switchable,
 	} PrimaryDisplay;
 	u8 SkipExtGfxScan;
-	u8 ScanExtGfxForLegacyOpRom;
 
 	/* GPIO IRQ Route  The valid values is 14 or 15*/
 	u8 GpioIrqSelect;