Remove Kconfig variable that has no effect

MAINBOARD_HAS_BOOTBLOCK_INIT is only declared once and selected elsewhere
(with no overlap), and never read. Remove it.

Change-Id: Ica1f16182b556dbf4a3b747237af74bcc4c0608c
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10254
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/arch/riscv/Kconfig

@@ -24,7 +24,3 @@ config ARCH_RAMSTAGE_RISCV
 config RISCV_BOOTBLOCK_CUSTOM
 	bool
 	default n
-
-config MAINBOARD_HAS_BOOTBLOCK_INIT
-	bool
-	default n