target/riscv: Remove redundant "cpu_vl == 0" checks

Remove the redundant "vl == 0" check which is already included within the  vstart >= vl check, when vl == 0.

Signed-off-by: Nazar Kazakov <nazar.kazakov@codethink.co.uk>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Max Chou <max.chou@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230711165917.2629866-4-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>