hw/riscv: Remove macros for ELF BIOS image names

hw/riscv: Remove macros for ELF BIOS image names

Now that RISC-V Spike machine can use BIN BIOS images, we remove
the macros used for ELF BIOS image names.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>