mb/google/nissa/var/joxer: Remove VBOOT_GSC_BOARD_ID config

Board IDs are now filled in as part of the signing process, so we don't
need to set them in coreboot.

BUG=b:240620735
TEST=Build and check VBOOT_GSC_BOARD_ID is set to ZZCR.

Change-Id: I7dda8ad59046a1dd9a28595e037eda86e91c98df
Signed-off-by: Reka Norman <rekanorman@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75641
Reviewed-by: Himanshu Sahdev <himanshu.sahdev@intel.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brya/Kconfig

@@ -292,10 +292,6 @@ config VBOOT
 	select VBOOT_EARLY_EC_SYNC if !BOARD_GOOGLE_BASEBOARD_NISSA
 	select VBOOT_LID_SWITCH
 
-config VBOOT_GSC_BOARD_ID
-	string
-	default "LBTV" if BOARD_GOOGLE_JOXER
-
 config DIMM_SPD_SIZE
 	default 512