mb/system76/lemp9: drop ONBOARD_VGA_IS_PRIMARY

Drop config ONBOARD_VGA_IS_PRIMARY as it's only needed for mainboards
with multiple graphics devices.

Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Change-Id: I6525c65af3dcfc96ea3d68a1388432179e9ac43d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43636
Reviewed-by: Jeremy Soller <jeremy@system76.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/system76/lemp9/Kconfig

@@ -53,10 +53,6 @@ config CONSOLE_POST
 	bool
 	default y
 
-config ONBOARD_VGA_IS_PRIMARY
-	bool
-	default y
-
 config UART_FOR_CONSOLE
 	int
 	default 2