target/riscv: Remove helper_set_rod_rounding_mode

target/riscv: Remove helper_set_rod_rounding_mode

The only setting of RISCV_FRM_ROD is from the vector unit,
and now handled by helper_set_rounding_mode_chkfrm.
This helper is now unused.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230115160657.3169274-3-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>