target/riscv/vector_helper.c: clean up reference of MTYPE

target/riscv/vector_helper.c: clean up reference of MTYPE

There's no code using MTYPE, which was a concept used in older vector
implementation.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-Id: <20230608053517.4102648-1-xiao.w.wang@intel.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>