arch/riscv: Don't link `stages.c` into ramstage

It's superseded by `ramstage.S`.

Change-Id: I81648da2f2af3ad73b3b51471c6fa2daac0540b1
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36610
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/riscv/Makefile.inc

@@ -140,7 +140,6 @@ ramstage-y += fp_asm.S
 ramstage-y += misaligned.c
 ramstage-y += sbi.c
 ramstage-y += virtual_memory.c
-ramstage-y += stages.c
 ramstage-y += misc.c
 ramstage-y += smp.c
 ramstage-y += boot.c