hw/char: riscv_htif: Drop useless assignment of memory region

hw/char: riscv_htif: Drop useless assignment of memory region

struct HTIFState has 3 members for address space and memory region,
and are initialized during htif_mm_init(). But they are actually
useless. Drop them.

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: <20221229091828.1945072-4-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>