target/riscv/cpu.c: remove 'multi_letter' from isa_ext_data

target/riscv/cpu.c: remove 'multi_letter' from isa_ext_data

We don't have MISA extensions in isa_edata_arr[] anymore. Remove the
redundant 'multi_letter' field from isa_ext_data.

Suggested-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230406180351.570807-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>