target/riscv: Remove misa_mxl validation

target/riscv: Remove misa_mxl validation

It is initialized with a simple assignment and there is little room for
error. In fact, the validation is even more complex.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Acked-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240203-riscv-v11-1-a23f4848a628@daynix.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>