mb/google/brox/Kconfig: Don't redefine config option

Commit 9b230ae295 introduced a redefinition of the config option
`BOARD_GOOGLE_BROX`, which is already defined in Kconfig.name
accordingly and thus causing a Kconfig warning. Fix that by removing the
type redefinition.

Change-Id: Iea6219a686a23d8d48a0bfb6ac642efd482fded9
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78394
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brox/Kconfig

@@ -51,7 +51,6 @@ config BOARD_GOOGLE_BASEBOARD_BROX
 	select TPM_GOOGLE_CR50
 
 config BOARD_GOOGLE_BROX
-	bool "->  Brox"
 	select BOARD_GOOGLE_BASEBOARD_BROX
 
 if BOARD_GOOGLE_BROX_COMMON