libvirt_bench_dump_with_unixbench.py: remove useless code

the variable guest_unixbench_pids keeps None all the time,
so utils_misc.kill_process_tree(pid) never be called.
this patch used to remove these useless code.
Please remove the same variable in other place:
```
[root@yds-PC io-github-autotest-libvirt]# grep -r "guest_unixbench_pids" libvirt/tests/src/
libvirt/tests/src/libvirt_bench/libvirt_bench_dump_with_unixbench.py:    guest_unixbench_pids = []
libvirt/tests/src/libvirt_bench/libvirt_bench_dump_with_unixbench.py:        for pid in guest_unixbench_pids:
libvirt/tests/src/libvirt_bench/libvirt_bench_domstate_switch_with_unixbench.py:    guest_unixbench_pids = []
```