disas/riscv.c: Remove unused decomp_rv32/64 value for vector instructions

disas/riscv.c: Remove unused decomp_rv32/64 value for vector instructions

Currently decomp_rv32 and decomp_rv64 value in opcode_data for vector
instructions are the same op index as their own. And they have no
functional decomp_data. So they have no functional difference from just
leaving them as zero.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230523093539.203909-7-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>