hw/riscv: spike: Remove the out-of-date comments

hw/riscv: spike: Remove the out-of-date comments

Spike machine now supports OpenSBI plain binary bios image, so the
comments are no longer valid.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221229091828.1945072-9-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>