libvirt: remove useless code about guest_unixbench_pids[]

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.
Merge pull request #393 from PandaWei/redundant

libvirt_bench_dump_with_unixbench.py: remove useless code